At some point you'll need to make an absolute minimum payment of an unchanging amount.

Otherwise the postage will cost more than the minimum 3% payment to be made, for a long time.

Without a set-or-fixed minimum $ amount at some point the exercise is not useful.

It would be more meaningful to say that the minimum payment is 3% or $5 (or $10), which ever is greater. Then you have a believable predicament.