shipping cost probs yet again

sold 3 of an item with unknown shipping cost ( it depends a lot on how many and how you want them cut, eg 1 at 1m or 5 times 1m lengths cut at .333mm would prob cost the same due to packaging weight ).
set the shipping at £3.75 for all 3 and it showed on the screen to totals to be 3 of the item at £2 making £6 and £3.75 shipping totaling £9.75.
the emailed version of the invoice shows shipping as £11.25, when this has happened in the past going to the invoice on here shows a total of £9.75 but I still had an angry buyer thinking I was trying to rip them off for £11.25 postage for an item of a few grams.
somewhere in the invoice creation bit of the site it is multiplying the shipping on the emails but doing it right on the site.
can this be looked into please