I paid with Paypal, no credit/debit card needed.
Did you have enough money on the paypal account? I mean, my is always at 0€, as I expect that what I pay via paypal gets charged from my account. This could be a different approach I could try -- transmit the money to paypal and then trying to pay.
Basically, you buy a new $96/year option, after it goes through Paypal you're sent to a screen where you can enter an existing Astraweb username & pass (to top up credit) or create a new one. You'll also see your Paypal transaction ID. DON'T fill in these fields, just open a support ticket with Astraweb (or email) and ask for your account to be upgraded, quoting the username and transaction ID.
OK, I wanted to do that. The problem is, that after I log in paypal says -- argh -- now I am reading it, on the log in screen: "Abonnementzahlungen können nur über Kreditkarte gebucht werden."
No wonder afterwards I get: "Sie müssen eine Kreditkarte hinzufügen, um den Einkauf abzuschließen. Der Verkäufer akzeptiert ausschließlich Sofortzahlungen."
Still very strange that it worked for you.