Pick whichever you already have. Every source feeds the same invitation queue, with the same limits and the same deduplication by order number.
A user with webapi access in your store panel. We pull orders together with line items, so we can ask about specific products.
Read-only REST API keys. We take orders with the completed status — we ask after delivery, not right after checkout.
One required column: email. Several rows sharing an order number produce one invitation with a product list. Re-running an import is safe — duplicates are skipped.
A single POST with your API key once an order is fulfilled. Idempotent by order number, so calling it twice will not send your customer a second email.
A product reviews feed for Google — product stars in Google Shopping. You plug one URL into Merchant Center and Google fetches the reviews itself. All published reviews go into the feed, per Google policy.