PayPal is one of the options that is used by many of the online stores as part of checkout. I find that they run different deals so many times a year you can get an extra 5% off stacked

Earn $10 for every friend (link could help the site)

Your friends are our friends. When they join and spend $5, you both can earn $10. 

I've used it at  Canada's Wonderland, many Six Flags Parks, and even some business expenses