short update: we probably found the right way to take previous payments into account during your order in webshop: a voucher you apply to your order that adjusts the amount payable accordingly. Since the shop cannot guess what been your previous payments when you create your account, you will probably have to tell the shop about your previous payments during signing up to the shop (aka "create account") Please add precise info how to identify your previous payments, like the correct amount if possible date as well as payment method (PayPal or bank transfer) and for PP the email address, for bank transfer the sender's account info and/or precise subject. I will check these info against our database manually and confirm/edit.

update to the update: We possibly found a way to provide voucher codes to all our previous donors which allow reclaiming of previous payments right on first visit of shop