How can we improve WHMCS?

I suggest the following feature: if a client pays an amount less than what is stated on the invoice, it would be convenient for the unpaid amount to be charged to the next invoice. For example: the invoice is $15.70, but the customer pays $15, the remaining amount of $0.70 will be charged to the next invoice. The important thing would also be to configure a minimum amount available to leave unpaid.