I'm all for great customer service, but 6-9% discount rates are frightening. It's not that hard to do direct paypal/credit card processing for something in the 2.5-4% range. That's a big chunk of a company's revenue, not to be given up lightly.
Also, even in the US, you'll need to build a lot of code on your site to support all the options that FastSpring and other similar services provide, such as all the offers, discounts, cross-sales etc. It's possible to do and maybe even more integrated, but still it's additional work that needs to be done and maintained.