ChainNative Server is code
Most new merchants will likely only consider the price of the service. Since BTCPay Server is free, that may have led you here and if so, welcome.
BTCPay Server is code, not a company. There is no third-party between a merchant and a customer. The merchant is always in full control of their funds. There are no processing or subscription fees. BTCPay Server is free to use and completely open-source, so developers or security auditors can always inspect the quality of the code.
We want users to understand not only BTCPay Server, but also how payments can be processed on behalf of a merchant. In order to help users understand trade-offs when using various cryptocurrency payment processing methods. We don't know which payment processors provide which services. That will need to be part of your own research. The following list is a good place to start: