Questions relating to processing Bitcoin payments with BTCPay Server. Keep questions to those directly related to BTCPay Server and the BTCPay Server API. Questions relating to BitPay documentation are off topic.
BTCPay Server is an independent, free, open-source payment processor for Bitcoin and a limited range of altcoins.
The server can be self-hosted (typically via Docker containers) in a range of environments includes Azure, Google Cloud and Raspberry Pi. Alternatively there are various hosts that offer hosted instances of BTCPay Server.
As well as accepting standard Bitcoin, BTCPay Server also integrates with the Lightning Network and comes with a range of integration tools built in for popular platforms such as Wordpress, Drupal, WooCommerce and Magneto.
More information about BTCPay Server is available on their homepage, https://btcpayserver.org/.