Provision an AvaTax Cross-Border account in the Avalara Admin Console and retrieve your account number and license key for HTTP Basic Authentication.
Authenticate to the AvaTax REST v2 API using Basic Authentication with the header value Base64-encoded from accountNumber:licenseKey.
Build a createTransaction request body with type set to SalesOrder, set the isSellerImporterOfRecord flag appropriately for DDP vs DAP incoterms, and include each line item with its HS code (or Avalara product classification code), declared value, origin country, and ship-to country.
POST to the AvaTax v2 transactions endpoint; parse the taxLines array in the response to extract duty, VAT/GST, and any carrier surcharge components separately.
Display the itemised landed cost to the shopper; convert the SalesOrder to a SalesInvoice transaction at the time of purchase to commit the calculation for reporting.
Use the Avalara Cross-Border Restrictions API to check whether the destination country prohibits or requires a license for the product category before completing the order.
Known gotchas
Avalara AvaTax uses HTTP Basic Authentication with account number and license key, not an API key header; sending a Bearer token to this API will result in a 401.
Setting isSellerImporterOfRecord incorrectly changes who is liable for duties and which duty scheme applies; DDP (seller is IOR) vs DAP (buyer is IOR) materially affects the tax calculation.
AvaTax cross-border calculations are estimates unless Avalara Guaranteed Landed Cost is enabled; inform customers of this distinction to avoid disputes at customs.
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