I'm with you on the sandboxing issues and lack of means to contact users.

FWIW, Fastspring is fairly customizable! The value over Stripe is that you don't have to deal with PCI details or worrying about fraud. Also, taxes/VAT in various jurisdictions. (Disclaimer: current customer)

One minor con is that until your store is "vetted" (something like a few months of successful transactions), your checkout form will include mailing address and phone number fields for additional fraud screening.

It's great to have more options of course. I figure I will "graduate" to Stripe if my operation gets to the point where I want even more control.