An agent learning a payments API against production is a bad idea, and most vendors here have no test environment at all. PayPal's sandbox is a different host with the same shape, so the same client config works with one URL changed — which means the rehearsal is genuinely cheap. Do it: an invoice created by mistake is a customer conversation.
PayPal's hosted endpoint for merchant operations, with a matching sandbox address. Both SSE and streamable HTTP are supported.
The merchant toolset, including invoice creation; PayPal's agent-tools catalogue is the full reference. The endpoint requires authorisation before listing.
Client credentials from the PayPal Developer Dashboard exchanged for an access token, then a consent step. Their documented path is to test in sandbox before switching to production.
Paste a URL, then authorize — add the endpoint to your client, then approve the OAuth consent screen