Zero Dollar Shipments/AR Invoice Entry

Hi Experts!
I routinely create zero dollar order lines and generate pack slips for project shipments. We do this so we have a firm shipment date that we can use for reporting (rather than trying to capture dates on progress billing). How can I prevent these zero dollar shippers from appearing when AR browses for packing slips when generating invoices? When I first started this process a year ago, it wasn’t an issue. I haven’t changed anything in my process, but now those shippers are appearing as read to invoice. We’d like to prevent a zero dollar invoice from being generated and sent to the customer. It’s probably something simple, but I’m stumped. Thank you!