I'll always look for the best shipping options and you'll see them in the site with products and the cart.
Plastics are funny - I can ship ten bookmarks in an envelope if I dab them onto a sheet to keep them as flat as possible, but if they all bunch up then they have to be sent as a parcel. And some things like a moon lamp weigh a few hundred grams but the size means they go as a much more expensive parcel.
I hate postage so where possible I'll try for a very low postage cost. Sometimes (as with bookmarks, for example) I'll include postage in the price. When I do, I'l mention it in the product description.