It's fairly normal for Sixt in that given any opportunity they will try to convert a pay-later reservation to a pay-now reservation (often with cancellation penalties). Sometimes (but not always) they will entice you with a lower rate for pay-now. If you don't wish to pay up-front, be clear with them about that. They may push back, and even tell you they won't link it unless you pay now, but you can certainly ask. Lastly, if the price hasn't increased since you booked, you can just cancel and re-book while logged in with your Sixt member account.