Zipper is a free compression program, and you don't need to pay anything for it. It doesn't contain pop-up ads or other annoying things. However, Zipper isn't free to maintain and wasn't free to create, because it contains commercial components, and was build with programming software.
While Zipper is a free program, it isn't a cheap program, and rivals many commercial programs in simplicity and ease of use. When you download the program you will have the opportunity to make a small donation by PayPal if you choose.