User Activity

Had a message regarding payment changes, I started filling in the information when I was asked for my password and pin. I got so far and realised this is personal information not for anyone else. I didn't totally complete the infomation. Called my ba...
I thought as a seller I was the one to combine postage. Somehow the buyer did this. Not a problem as the correct amount was deducted but just wondering how this is possible for the buyer to alter an invoice.