You should have a feature in Essentials for when a customer overpays, the additional amount can be applied to their next invoice easily. At present there is no function to process an overpayment except to change the invoice amount temporarily so that the payment can be allocated, then you have to change the invoice back to the original amount.
It would be handy to have a facilty that stores the amount of credit owing to a customer and asks if you want to apply it when the next invoice is generated.
"Customer in credit"
