I'm pretty sure that you can only use a code or get a sales price. They dpn't let you stack discounts (at least I think). So if the sale price is more than 15% off they'd be better of just doing that instead of using the code.
Taylor and Misfit Momma are correct - the affiliate coupon code acts like other coupon codes, which can't stack on top of items that are on sale already. The code will take off 15% from regular prices, though, which is still a nice incentive to get someone to buy something.