You could ask WePay. I don't know that they have pledges now, but maybe they could be persuaded to add them. What you can do quite easily with WePay is to sen

You could ask WePay. I don't know that they have pledges now, but maybe they could be persuaded to add them. What you can do quite easily with WePay is to send bills by email, each including a link to the payment page with the amount filled in. This makes it pretty simple for people to pay at that point. But it isn't, admittedly, automatic, and some people could ignore the bill. I don't know how big a problem this would be in practice.