How to automate bitcoin wallet to send return payments to website subscribers?
How can a bitcoin wallet even be programmed to automatically respond to incoming payments with a return payment?

What is a bitcoin wallet, exactly? It can be just about anything that is programmed to create keys, check UTXOs and build/sign transactions... A bitcoin wallet can be programmed to do just about anything.
Your question is broad... You need to program it yourself. Build a database to keep track of addresses and payment codes and whatever else then build your site to interact with your database and your wallet, or build your own wallet.