Coupon codes for many products

I sell extensions. If I want to a have 20% off sale for all extensions, I have to add a coupon code for every extension. I would like to be able to apply one coupon to many products.

actually, I just tested and it won't even work that way. If I want to have a sale on all extensions and I have 10, how can user receive a discount on the whole purchase? Now it only applies 1 coupon and ignores others.... what can I do? This feature is much needed or I don't think can use this product. And I really like it.