Edit: While going to work, I thought about what I said. So I did some research and it indeed looks like you do require a full-node with BTCPay Server, but it comes with the installation. Sorry for giving you the wrong information initially.
Yes, it's required to have a full node with BTCpayserver but they also have options to use a 3rd party if you don't have a budget to build a full node.
@zkquality
If you are looking for 3rd party BTCpay hosts you can find them from the link below
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https://directory.btcpayserver.org/filter/hostsSome of them are free and paid but if you care about your privacy then you would need to build a full node.