Why don't you use a simple python script to derive addresses from an xpub?
Just create a new wallet with an SPV wallet like electrum or a ledger, export the xpub, move it to your server, then just derive a new address for each customer.

This way you won't rely on a thirth party, and you have full controll over everything that happens, while still having maximum security (since your xprv will never touch a webserver)