Cant buy SaT

[firgli]

Hi, I'm trying to buy stack and tile but for some reason nothing happens when I click buy, I tried both paypal and neteller. Paypal redirects me back to stackandtile where I can start all over again (while it looks like I paid, paypal acts as if the payment was succesfull before redirecting though I didnt pay anything). And it says my mastercard number isnt valid even though it is.
Any idea what could be causing this?

Thanks!