Clean up 'Mass Pay' invoices
If the client decides to use the 'Mass Pay' feature, but doesn't pay the invoice, the invoice will continue to be active. Let's say the client forgets about this invoice and decides to pay for each of the "child"-invoices manually - the mass pay invoice will still not get cancelled.
If the client has a credit card added, the payment for the mass pay invoice will automatically get charged 7 days after. Now, I would be able to live with that if WHMCS would add the charged amount as credit... but that doesn't happen. The credit is transferred to the "child"-invoices, but that doesn't give the client a positive credit balance.
I'm suggesting that as soon as a client pays for a "child"-invoice, all mass pay invoices relating to that "child"-invoice will get cancelled.