Did you know, that a buyer of your item can change your invoice?
Some people keep this preference on if they are offering discounts on thier items. But beware! A buyer can change other things too...like shipping cost and the cost of insurance on an item, even if you have stated that insurance is required. A simple fix to this is, changing your preferences. Heres how!
Go to your my e-bay page
Click on preferences (left hand side of your screen)
Under the heading Payment from buyer click show
Change Allow buyers to edit payment totals from yes to no
There are many other preferences that you may want to look through and make sure it's what you had in mind.
Good luck with all your sales!