Why can’t you cover postage costs for items being returned?

I based my pricing on the majority of orders being for 1 or 2 picturecards only, and in contrast to many e-commerce sites, I refuse to charge upwards of £2.95 (UK delivery alone) for such non-bulky mail. This saving means; in the unlikely event of having to return an item, the cost of doing so is still decent value in comparison with this typically high delivery fee.