Avalara AvaTax vs Zamp

Avalara AvaTax

The REST API has a public Swagger file with 491 operations, a Markdown page per endpoint and a 90-day trial with no card. Production rate limits and prices per transaction are not published, the API patch notes stop at April 2023, and the tax calculation service was degraded for about 23 hours on 5 and 6 October 2026.

Zamp

A free 30-day sandbox key needs no card or sales call, and the docs are served as Markdown with llms.txt and a published limit of 1,000 requests a minute. There is no OpenAPI file, SDK, API changelog or published price, production keys come with a sales-led account, and the site footer says SOC 2 compliance is in progress.

Avalara AvaTax

1. Omit type or send SalesOrder for an estimate. A type ending in Invoice is recorded, and commit set to true locks it for reporting
2. Retry a timed-out invoice with CreateOrAdjustTransaction, not CreateTransaction, which errors when a committed transaction has the same code
3. Use sandbox-rest.avatax.com with sandbox credentials for tests. Production is rest.avatax.com and the two accounts are separate
4. Add $include=SummaryOnly or LinesOnly to shrink transaction responses, and page lists with $top (1,000 at most) and $skip
5. Call /api/v2/utilities/ping first. It answers without credentials and shows whether the credentials sent were accepted

Zamp

1. Call POST /calculations for a quote and POST /transactions only when the sale is final. Put the returned taxDue in taxCollected when posting
2. Keep transaction and line item ids to 25 characters. A repeated POST /transactions with an existing id returns 409 CONFLICT, so a retry doesn't create a second record
3. For a refund, post a new transaction with parentId set to the sale's id and a negative quantity. Never send a negative amount on a line item
4. Send shipToAddress.state and country set to CA for Canadian orders. POST /addresses validates US addresses only
5. Page lists with limit (1 to 100, default 10) and cursor until nextCursor is null. GET /transactions returns only transactions written with the same key
6. Don't store sensitive or personal data in the sandbox. Trial data is deleted after 30 days and the trial terms forbid live customer data