I have sold on ebay and never encountered this problem with any customers.
I have also bought on ebay and HAVE encountered this problem several times with other sellers.
When invoice comes it will show shipping charges (even though it's free).This has also happened with being taxed from other states (illegal !! )
I send a message to adjust invoice properly so as I may pay or I will contact ebay to remove my bid.
It is an extra step but I figure how bad do I want what I bid on.
Has worked out to satisfaction each time.
Since I am not sure where the problem came from
I just don't leave feedback in those cases.
I don't wan't to blame some one if it was out of their control; which does happen.