Calculate sales tax using Avalara AvaTax CreateTransaction API

Steps

1. Authenticate using HTTP Basic Auth with your Avalara account ID as the username and license key as the password (Base64-encoded in the Authorization header).
2. POST to https://rest.avatax.com/api/v2/transactions/create (or the sandbox equivalent at sandbox.rest.avatax.com) with Content-Type: application/json.
3. Build the request body with type (SalesInvoice or SalesOrder for a quote), companyCode, date, customerCode, and an addresses block specifying the shipFrom and shipTo addresses with street, city, region, postalCode, and country.
4. Include a lines array where each line has number, quantity, amount, itemCode, and optionally taxCode (Avalara System Tax Codes) for accurate product-specific tax treatment.
5. The response contains totalTax, totalAmount, and a lines array with per-line tax detail including taxableAmount, tax, and rate, as well as a summary of tax authorities.
6. To commit the transaction (for reporting/remittance purposes), either set commit: true in the request or issue a separate POST to /api/v2/transactions/{companyCode}/{transactionCode}/commit.