This has been partially discussed in other threads; however I think we have a bit of a different take on it.

If you create a PO for say 20 of an item A. The supplier only has 15 of each so you get a delivery of the 15 with another 5 on back order. You can accept in the 15 delivered items, which creates 2 line items currently in the PO, however the PO cannot be Finished, which sounds reasonable, but it is a problem.

The suppliers will send us the 15 delivered on one Invoice, and when the remaining 5 are delivered they will send a second Invoice for the remaining 5.

You need to "Finish" the PO for RepairShopr to generate the invoice on the QuickBooks Online side.

If RS has an ability to Back order status items, or allow the remaining items on the PO to be split into another PO that would allow the system to pass this roadblock.