The relevant docs for the BTCPayServer plugin are here: https://docs.btcpayserver.org/ShopifyV2/ - I have been looking a bit at the BTCPayServer code... the "back end" is a C# plugin.. https://github.com/btcpayserver/btcpayserver-shopify-plugin/tree/master/Plugins/BTCPayServer.Plugins.ShopifyPlugin .. then there is a small node.js app that constitutes the "Shopify Checkout Extension" here: https://github.com/btcpayserver/shopify-app ...

It might be straightforward to study this docs page and these two github repos, and they would tell you most of what you need to know to build this.. This is something I would like to work on, likely won't have time for a few weeks but it seems like somebody should build this, because it could be really powerful, right?