11:43 Nov 7 

Yes, the insert is the blanket; (I used to design shams and duvet covers), anything that goes into a cover and goes on a bed is an insert, in this case a polyester blanket.

10:39 Nov 7 

It really all depends on that, but from the text given, it does indeed sound to me as if these 'inserts' are the polyester blankets themselves. Perhaps they are 'inserted' into a cover of some kind, just as a 'couette' would be insered into a 'housse'?

It's a not uncommon usage of 'insert', which is often applied to things like cushions, pillows, etc., specifically when referring to them in comparison with the outer or cover.

e.g. "The cushion comprises a canvas cover and a foam insert"