As mentioned on the Registration Wizard page, you cannot use a disposable e-mail address.
1st: Why did you accept it in the first place when it comes to pay?
2nd: @will-hier-weg.de is not a disposable e-mail address. It it provided by gmx and is a fully-functional mail address.
I can not check, how good kvs is, because, from the decision, to use your tool, until now, I lost my free weekend day.
Consider following:
^ Try to gather less data of your customer. All you really need is money.
^ Shorten the time between first hit of your homepage and use of kvs to the absolute minimum. Why should it need your manual interaction.
This should be the next features, because functionality is one thing. But usability finally sells as well.
BTW, yes, I got my access, 15 hours it took.
Okay, I gave the other address, and the credit card seems to be refused.