the point is you are suppose to have correct postage in the listing. You are not allowed to add on postage if you miscalculate it. I would be peeved to if I was the buyer.
Get a set of scales, weigh the items and set up your postage as calculated. The buyer will then see the postage based on their location.

Some of your postage is way expensive. You could send a lot of what you are selling for $7.50 + 1.20 tough bag with some bulbble wrap.