Correct Way For Clerk To Determine Postage Due Amount

At home, I put some stamps on my mail for Japan. Then I go to the PO and have the clerk weigh it and subtract the postage I've already affixed. Today the clerk used scraps of paper and wrote the amount of each stamp on my parcel and with some difficulty attempted to add the column. On my receipt there was no record of the stamps already on my package, only the final amount she charged me. I feel very uncomfortable with this. At a different branch, the clerk  has used a calculator and showed the amount subtracted on my receipt. Please tell me the correct USP procedure. As well, the clerk today was rude to me when I suggested she use the calculator. Thank you for answering this question.