Just took a quick look. Seems that Coinbase can serve as a bitcoin payment gateway and they have what appears to be a mature and well documented API: https://developers.coinbase.com/docs/wallet/guides/send-receive
Could you use this?
That’s as simple as posting a Bitcoin address that you control.
Not needing a payment service provider or a payment gateway to send or receive payments is after all *the* central idea of Bitcoin.