A lot of sellers give a discount on subsequent items to make up for the extra postage. I found capped postage saved me having to do refunds. I added about 50c to every item and that more than made up for any extra postage I might have to pay.
I send letters but you could probably work out something for parcels, eg. add up to 50c to all your prices and then cap postage at a small satchel. If they buy several at once the extra 50c on each item should cover any extra postage. It works for some things but not others, and works better for small(ish) items, but you may have to average some things out.
