AFAIK, open source wallets allow you to request to receive a certain amount of Bitcoin or its equivalent in dollars, but the prices will be fixed.
Therefore, you need to rely on third-party applications, which are either closed source or open source.
BTCPay is the best option, it is free/open soure and does not require much time, you need to extract your Master public key.
If you are looking for a payment gateway for Bitcoin with 0% fees the best option for you is BTCPay. You have an option to create a
payment request/invoice(video demo on the link) which you can send to your buyer where he has the option to copy the address or scan the qr code. If you know how to work around with your website with various codes and you want to integrate a payment system on it this I think is one of the best options for you.