If you'd rather not use a credit card, PayPal is offered as a billing option for Dropbox Pro accounts.
German PayPal users have reported problems with recurring payments. If you use a German bank and are attempting to bill monthly, your bank may decline subsequent monthly charges after the first successful charge. PayPal has acknowledged the bug and is working to fix it. In the meantime, you may want to consider changing your billing options to an annual payment plan to avoid having to re-enter your billing information every month.