Postal charges?

As happens on occasion we had a notification through the door that there was insufficient postage for an item. As I say this happens and I'm used to see the differences because a letter, large or otherwise, stamped for 100g weighs 101g on the Royal Mail's scales.

This time the difference was 85p; "Odd" I thought. The method used (from experience) is to take the second class stamp cost for the correct band and subtract postage already paid. So a Large Letter under 100g of 66p turns out to be in the next band, the 2nd class stamp cost is 81p. 15p is owed. The Royal Mail then add a £1 handling fee.

I've got the fees in a spreadsheet already and I checked they were up to date so it was easy to run a calculation on 2nd class minus each price of the previous bands. None produced a figure of 85p.

I put the notice to one side to pick up on Saturday (the office is open 7-12:30; so I'm either leaving early or trying to do it during the peak traffic period) and then forgot about my concern when I picked it up (I was also in a rush and parking was a problem)

It's a Large Letter weighing about 120 grams with a first class 'Large' Stamp worth 66p on it. Price difference, as I said, 15p not 85p.

Not my problem, it's from a company we deal with so I'll invoice them and they can argue with them; but still worth paying attention to these things.