BTCpay Server is a really cool project, it’s an open source replacement for BitPay. It’s used in many sites, including some popular ones like namecheap.com, and it also integrates with quite a few popular commerce platforms like Wordpress, WooCommerce, Drupal, Magento and Shopify.
The core developers only support bitcoin development, but it’s made to allow third parties to provide support for other coins. Currently it supports a dozen or so including Litecoin, Dash and Monero.
I’d love to see Cardano support out of the box in BTCpay Server and would be happy to support the development of this depending on the estimated size of the job, which could be discussed here by anyone interested in the idea.
Their Alt coin FAQ covers the details of adding support for new coins.
Also it might be worth noting that all the supported alt coins (I think) are bitcoin forks except for Monero, so this may be the best one to look at for an existing integration example. Monero seems not to have needed to include support for NBitcoin and NBxplorer like the other coins have.