Fritz, thanks for the reply and your suggestion. But I don't think it is a feasible option in my situation.

I just tried using the Estimate approach. However, the email received is just a simple text message. It doesn't include buttons which would allow the member to make a payment at this step.  So if I understand correctly, the estimate approach would generate an email to the member. They would then need to reply to the email with the amount they intended to pay. Then I would need to convert the estimate to an invoice and sent it back to them.  Seems like this would add an additional step to the process I was already considering. I also think it would confuse many of our members, not to mention the work to reply to and track over 500 emails and replies (that is the number of potential members we have)

Again, thank you for the suggestion.  It is helpful to now understand how estimates work.