I spent some time searching around for a mod, feature or fix for Marketpress that would allow coupons to work just for specific items, rather than the whole store. Not finding one, I figured I'd just do it myself.
Also having found a bunch of requests for similar features, I figured I'd share my beta code with y'all. The current version, which just allows coupons to be applied to certain item IDs, is here: http://pubcdn.pottervilla.com/marketpress_per-item_coupon_mod_0.5.0_from_marketpress_2.8.7.zip .
All my edits are tagged with the initials JLP and the id mp_itemspecific_coupons . There's currently 8 edits. This is a modified version of the marketpress.php file, it should work if you just drop it in place of marketpress.php in a 2.8.7 installation.
Feature wise, I just added a column on the products > store settings > coupons page for allowed product IDs. You can create or edit a coupon there, and add any number of comma separated product node IDs (I usually pull them from the edit URLs), like 60,14,23,112 .
When your customer checks out, if they put in that coupon without having a valid product in their cart, it will respond that the coupon is invalid. If they do have the product in their cart, it applies the discount. If the discount is an amount, it's applied straight up. If the discount is a percent, it only applies to the valid items.
The whole mess here was coded up in a couple hours, and (so far) works great on my site. If anyone out there wanted to test further, I'd be happy to hear about bugs.
I'm just using this on a small site, and don't need category based coupons, but may add that if I end up using it on a bigger site.
Yeah, so I think that's about it. Thanks!