There are just few BTCpay server instances available for merchants to use. The ones I found are:
- https://mainnet.demo.btcpayserver.org/ managed by @BtcpayServer
 - https://btcpay.btc.aw/ managed by @BitcoinAruba
 - https://btcpay.cubabitcoin.org/ managed by @Cuba_BTC
 
We have been testing all of them with our SWAG shop. There was another in Canada, can't find it anymore.
Usually we tend to install and run BTCpay server for personal/single-access use. These instances above have publicly made the decision to enable access for others to register and use the server, with no extra cost added, simply to support their surrounding community and promote bitcoin adoption.
I think it is a great service for the community and there must be more, because merchants and businesses we talk with often are not tech literates, simply sue phone and PC. But asking to install a software in a server is kind too much. They just want to run their business. Accept bitcoin? Sure... how? Too complicated or technical? They probably skip and miss the opportunity.
Should be there more instances available for businesses and vendors?