FDM shipping credit

Every time I’m scanning something on amazon to work fees and profit etc it always pops up merchant before amazon. I’ve always noticed there’s + for shipping and its always around £4.48/£4.49 without including the what the profit sells for (even on items that sell for less than £3 for example) can someone explain this ? Like 4.50 credit seems a lot for something that could fit in a letter.