Backorder is just another word for Print-on-Demand in the Ingram side of things.
This is because there are never any books sitting around waiting to be shipped off.
They are produced upon ordering, and then sent off. We understand how this can be confusing sometimes, think of 'backorder' as a synonym of Print on Demand, and it makes better sense.
You can advise the bookstore of this information. Once they realize 'backorder' is not a true problem and they continue the ordering process, they find there is nothing blocking them at all, it's just a 'head's up' so to speak.
If that does not resolve this for you, (but it should!)