For email forwarding, you can have a look at https://github.com/danisola/forward-cat. However, I do not know if this would work for your setup. On top of that, I do not think it handles replies to the email alias, but I am not sure.

What could work is to just use an internal system for messaging, so that each of your users has a private inbox in your store platform. Kind of like direct messaging in a forum...