How can we improve WHMCS?

Currently, when a customer or admin wants to pay multiple invoices with one payment, WMCS creates a new invoice - which is confusing for everyone!

Change to a system which just applies part of the payment transaction to each invoice and stores the record separately as a single transaction paying multiple invoices. So admin or client can enter a mass payment and just tick the invoice to apply that payment against, perhaps with a button to "auto apply" to assign parts of the total payment to oldest invoices first until the balance of the payment is used up.

"Mass Pay" invoices is the number one gripe from customers, as it just doesn't make any accounting sense for this to be an "invoice". At the very least, creating a new category of "invoice" and marking it "Mass Pay" in the list so that it's distinguishable from actual invoices would be better.