I don’t know if ya’ll had a chance to catch it but overpayment credits recently went live. You can save overpayment credits to customer profiles and then apply the credit to other invoices.
This is a good start - here’s a great way to polish that feature and finish it off the right way. Allow us to add a credit to a customers account. Sometimes a refund is not needed, sometimes we want to do something for the customer and this would be a great way to do that. “Hey thank you so much for the referral, we added a $200 credit to your account that you can use towards anything on your next visit” etc.