I have an online store, using zencart and it can manage that; once a purchase is to be made, it asks to log in, so it can double check. In S2M, it does not take it into consideration and requires the customer to know they have to log in BEFORE they fill out the form. So, having to rely on the customer to know what to do, when they have a form right in front of their eye to fill BUT they should go log in first and then come back, is NOT intuitive to all which is why i would offer that suggestion.
I have explained that inconvenience in details to Jason and he agreed that it could be improved. As far as a coupon with a button, maybe a popup could be added to the button to enter a coupon code, OR have a matching button saying “do you have a coupon code?” that the user could click to get a popup. Something like that. I am not a programmer, but a user, and i see a need as a user. I just rely on the programmer to see how to improve the product.