Just some small info regarding cushion inserts and the best alternative that I think you should choose:
Polyester is a plastic fibre mix which is a cheaper alternative to the feather or down mixture for cushion inserts. I will offer a polyester cushion insert for my cushion range but I dont recommend it as the best option.
You can not wash a polyester cushion insert as it will go lumpy and you wont ever get it back into that plump state. After time the polyester cushion also becomes quite flat and lifeless and wont show off the design as well as what it did when you first bought it. They are a lot lighter and softer, and you sort of dont feel as if they are there when you lean on them.
The feather and foam mix which is a more expensive option is better because of its quality. You will have this insert a longer lifetime then the polyester option. It is firmer and plump and can be moulded easiely back into shape. It can also be washed and the shape wont change, and it wont become flatter with age. The cushion will always look like a new feature wherever it is placed.
So when choosing a cushion insert think first about the quality and how long you will be wanting this cushion displayed in your home.