This task is a pain !!!!
Here's what I do, posted in the hope it will help someone.
I include the packaging cost in the price of my item - all are BIN
Then I put in the appropriate RM cost exactly, e.g. £4.41.
When April 6th comes round I download all my listings [call it spreadsheet A], and make a second copy of the spreadsheet [call it B].
Then on 'B' I do a 'Find & Save' on the postage columns updating to the new cost.
On the original spreadsheet, 'A', I change the number of items in each listing to 0 and upload the file [effectively deleting all the listings.]
Then upload the new spreadsheet 'B' and the listings are done and dusted.
Sounds complicated but works a treat!