It looks as if you are using a flat rate which means that all your US buyers pay the same shipping rate regardless of where they are in the US. If you use calculated shipping the buyer will be charged based on their zip code and the information that you have entered on the package weight and size. That is the more accurate way of charging but some do prefer to use a flat rate.
On one of your listings you are charging $14.95 so the buyer pays that amount regardless of what you have to pay for a label. The buyer pays you for the label and then you pay ebay if you are using ebay labels. So if the buyer pays $14.95 and the label is $20, you pay for the extra $5. If the label is $10, the extra 4.95 goes to you. Keep in mind that you also have packaging costs and that the final value fees are also charged on the amount that the buyer pays including shipping and taxes. That's why some sellers will charge their buyers the retail cost of the label even though they pay ebay's commercial rate. They use the extra for packaging and fees.
If you use an ebay label and for some reason the weight or dimensions are wrong and the postage you paid is actually higher, USPS will bill ebay the extra amount and then ebay will bill you. Your buyer will not get charged if the postage is underpaid IF you are using the labels here.
