Customers would like to add tips when paying in Client Portal

Customers would like to add tips when paying in Client Portal

I am happy with the clean interface of the Client Portal. However, I am running into a challenge: my customers would like to be able to add tips/gratuities when making payments. Currently, this is very clunky because they have to 1) manually over-pay, 2) let me know that the excess payment was a tip and not a pre-payment for the next service, and then 3) I have to go in and adjust the Invoice after the fact. These steps are awkward and messy and time-consuming!

My request/idea/resolution:
I would love to see an option for me (business) to be able to let clients click to add a "gratuity" line-item to an invoice so they can designate that an excess payment is intended as a tip, not a pre-payment or accidental overpayment.