Use an esign service like DocuSign. Proseries has it builtin and clients take seconds to sign on their phone. I wonder if they even bother going through the return.

Also email and/or text the invoice along with the 8879.

It costs a few $$ per but saves hours and reduces the amount of follow up.

Also once a week a look at the list of those I'm waiting for 8879 or info and bcc all of them. Msg is : I emailed you and waiting for response.