If you go to the listing on AX, right around the price there’s a… “pulldown”(??)… which says something like “Seller’s coupons”.
Force of habit, I just go and grab everything, and on checkout it automatically applies whichever is the biggest value.
Eg, if you grab “1buk off for orders over 15”, and “2bux off orders over 20”, and you order something for 22bux, it’ll pick the latter. Pretty kewl.
Also, if you have identical coupons, but different expiration dates, it’ll pick the one that expires sooner, letting you keep the other one for longer.