I sell mainly jewellery which is not eligible for SD.
After reading your post, I looked at my rating and it is 4.8 for postage costs.
I always combine postage if a buyer has more than 1 one item. If I send items 48 hour tracked, I charge £3.05. The cost at the Post Office is £2.80 so 25p goes towards a new padded envelope, good quality bubble wrap, tape etc.
But clearly a fairly high proportion of buyers think they are being overcharged.
Your scores are fine. I really would not stress about it.
I am sure if you hand delivered everything on a silver platter, some buyers would still not be satisfied!
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)