Whilst the feature is not available with on-line payment processing,  there is nothing in any AP terms and conditions that prohibits purchasing extra cover for ebay satchels, and according to the terms and conditions of Click and Send (which ebay satchels fall under) you should be able to.

Of course, as many of the AP reps are not aware of AP product T&Cs it is a problem - as my guesstimate would be that 2 out of 4 AP reps will say you can't purchase extra cover with flat rate ebay satchels.