Screendump below is my shipping setup.
One sticker/decal is about 3 grams which would result in € 0,96 shipping cost.
How ever, because of the envelope the weight will be 70 grams (which I put on the product) and that results in € 2,88 shipping cost.
Now if someone wants to order 2 stickers, the weight will become 2*70 gr = 140 gr and € 3,84 in shipping.
I can however fit 10 stickers in one envelope easy and shipping would be €2,88.
Using dimensions will not help as all 5 shipping methodes are for the same dimensions.
Hope this clarifies better.