When making a payment, a list of all open invoices is shown with an "Amount to Apply" box next to each one.

There should be a button to "Apply full amount" to a given invoice, and also an option to apply the full amount to all invoices.

This will save a lot of time when a client writes one check that covers all of their open invoices. Right now you have to either copy and paste the "invoice balance due" into the "amount to apply" box, or type it out.