It isn't the kind of experience we want you to come across, lmoorecalvarydevsvcs.

I can see the efficiency of adding a custom field to the invoice email. While I'm unable to provide an exact timeframe on when the feature will be implemented, you may visit our Feedback page to know the latest updates.

In the meantime, you can enter the additional customer requirements in the Subject line prior to sending the invoice.

Also, if you want to let your customers pay their invoices online, you can send online invoices. Here's an article as your guide: Take and process payments in QuickBooks Online with QuickBooks Payments.

I'm always around whenever you have questions on how you can manage customer transactions.