paying postage

1 / I have just sold an item but  the buyer made a slight misstake on the address.

i had already sorted the postage that Ebay sorts, but when i amended the address ( I ) had to pay the  postage .

2/ why doesnt Ebay have a check system to make sure the buyers  address is correct and in the correct format before the deal is set? or does it?

Many thanks in advance.