Hi,
I'm developing the paypal plugin. I didn't think of discount codes because it's such a pain in he ass to made them, but eventually you'll have them. What we're going to have is a in-website credit syste, in which each user could have a "wallet" and buy some credit in advance. We think of this, because paypal took like $0.34 + 2% on EACH transaction, If you're doing $2 payments (you'll receive like $1.5), paypal will took a lot from you (I'm not sure what happen with even lower prices). So you let the users the option to buy a $10 package for example (you'll receive like $9.4). For users is also more comfortable to not pay each time, instead, you make a payment and you have credit for several ads.
Discount codes will be includes, but unfortunately not in the first release.
Thanks for your suggestions.