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Building e-commerce platforms, SaaS applications, and marketplace solutions means navigating one of the most complex regulatory landscapes in software development: sales tax. Unlike federal income tax with consistent rules, sales tax operates across 47+ states and local jurisdictions, each with unique requirements, thresholds, and definitions.
This guide covers the fundamental concepts every developer needs to build tax-compliant systems. For implementation details, see our API Reference.

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The Four-Factor Taxability Framework

Sales tax isn’t a simple yes/no decision. Every transaction requires evaluating four interconnected factors:
Does your business have a connection to the state?Without nexus, you have no tax collection obligation in that jurisdiction. Nexus comes in two forms:
  • Physical nexus: Offices, warehouses, employees, inventory storage
  • Economic nexus: Sales volume or transaction count thresholds
Remote employees create physical nexus in their state, even if they work from home.
Is your product or service taxable in that state?Taxability varies wildly by state and product type:
  • Always taxable: Electronics, clothing, furniture
  • Often exempt: Groceries, prescription drugs, medical devices
  • State-specific: Clothing (exempt in PA, taxable in CA)
Does the customer qualify for an exemption?Common exempt customers:
  • Government agencies
  • Nonprofit organizations (501c3)
  • Businesses purchasing for resale
  • Educational institutions
Exempt sales require valid exemption certificates. Collecting tax on exempt sales creates audit liability.
Which location determines the tax rate?Two sourcing methods exist:
  • Origin-based: Rate based on seller’s location
  • Destination-based: Rate based on customer’s location

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Nexus Types: Physical vs Economic

The 2018 Wayfair Supreme Court decision fundamentally changed sales tax obligations, creating two distinct nexus types:

Physical Nexus

Traditional connection to a stateTriggers:
  • Retail stores or offices
  • Warehouses or inventory storage
  • Employees or contractors
  • Trade shows or pop-up shops
Timeline: Immediate upon establishment

Economic Nexus

Sales volume-based connectionCommon Thresholds:
  • $100,000 annual sales OR
  • 200 transactions per year
High-volume states:
  • CA, NY, TX: $500,000
  • NY: $500,000 + 100 transactions
Timeline: Register with effective nexus start date, start collection
1

Monitor Sales by State

Track both revenue and transaction counts for each state where you sell.
2

Set Up Threshold Alerts

Configure notifications when approaching nexus thresholds.
3

Register Before Collecting

Never collect tax without a valid permit—it’s illegal in most states.
4

Begin Collection Immediately

Start collecting tax on the very next transaction after registration.

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Sourcing Rules: Origin vs Destination

Once you have nexus and a taxable product, you need to determine which tax rate applies:
Tax rate based on seller’s locationStates: Arizona, California, Illinois, Mississippi, Missouri, Ohio, Pennsylvania, Tennessee, Texas, Utah, VirginiaHow it works:
  • Austin, TX merchant charges Austin rate to all Texas customers
  • Same rate whether shipping to Dallas, Houston, or rural West Texas
  • Simplifies calculation but concentrates revenue in business locations
Tax rate based on customer’s delivery addressHow it works:
  • Los Angeles merchant charges San Diego rate for San Diego delivery
  • Must calculate different rates for each delivery address
  • Navigate 11,000+ tax jurisdictions across the US
Hybrid approach with complex rules
  • State, county, city taxes: Origin-based
  • District taxes: Destination-based
  • Creates exceptional complexity for developers

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Marketplace Facilitators

E-commerce marketplaces have spawned specialized legislation that shifts compliance responsibility from sellers to platforms:

Marketplace Facilitators

Platforms that handle tax collectionExamples:
  • Amazon (FBA sellers)
  • eBay, Etsy
  • Walmart Marketplace
  • TikTok Shops
Responsibilities:
  • Collect and remit sales tax
  • Act as “retailer of record”
  • Handle all tax compliance

Non-Facilitator Platforms

Platforms where sellers handle their own taxExamples:
  • Shopify, WooCommerce
  • Wix, Squarespace
  • Custom-built solutions
Responsibilities:
  • Sellers must register and collect
  • Platform provides tools only
  • Full compliance burden on seller
Key Distinction: If you sell exclusively through facilitating platforms, you typically don’t need separate state registrations—unless you have physical nexus or make direct sales outside the marketplace.

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Collection Timeline

The timing of sales tax obligations follows a precise sequence that systems must automate:
1

Home State Registration

Register before making your first sale. Most states require permits regardless of volume.
2

Physical Nexus Registration

Register within 30 days of establishing physical presence (office, employee, inventory).
3

Economic Nexus Monitoring

Track sales by state and register within 30 days of exceeding thresholds.
4

Begin Collection

Start collecting tax immediately upon receiving your permit.
5

File Returns

File and remit taxes according to assigned frequency (monthly, quarterly, annually).

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System Architecture Requirements

Building tax-compliant systems requires several critical components. Kintsugi manages all of the below logic for you:
Continuously monitor sales data across all states, comparing volumes against current thresholds and alerting when registration becomes necessary.Key features:
  • Real-time sales aggregation by state
  • Threshold monitoring and alerts
  • Registration deadline tracking
  • Historical data analysis
Map inventory SKUs to state-specific tax rules, accounting for exemptions and special rates.Key features:
  • SKU-to-taxability mapping
  • State-specific product rules
  • Exemption handling
  • Regular rule updates
Determine appropriate rates based on precise geocoding of delivery addresses, distinguishing between origin and destination states.Key features:
  • Accurate address geocoding
  • Origin vs destination logic
  • 11,000+ jurisdiction support
  • Real-time rate updates
Collect, validate, and store documentation for exempt sales with built-in workflows for renewal and expiration.Key features:
  • Certificate collection and storage
  • Validation and verification
  • Renewal tracking
  • Audit trail maintenance
Clearly separate facilitated from direct sales in reporting systems.Key features:
  • Sales channel identification
  • Separate reporting streams
  • Compliance tracking
  • Audit trail maintenance
Maintain detailed records of every tax calculation, including nexus determination, product taxability assessment, rate sourcing, and exemption application.Key features:
  • Complete calculation history
  • Decision point logging
  • Data integrity checks
  • Compliance reporting
Sales tax compliance evolves as businesses grow, states change laws, and new sales channels emerge. Understanding these fundamental concepts—taxability factors, nexus types, sourcing rules, marketplace facilitator models, and collection timing—enables you to architect systems that scale with business growth while maintaining compliance across jurisdictions.

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Next Steps

Get Started

Ready to implement sales tax? Start with our Getting Started Guide and API Reference.

Need Help?

Have questions about your specific use case? Check our Support Center or contact our team.