My experience differs from yours in that Bandcamp sellers charge only what their local carriers charge, which is dictated by their location and because they are small independent labels without the advantage that comes with large volume. American labels are expensive, because they ship with USPS from USA. German labels are cheap. UK labels are fair to middling, but getting dearer. Italian labels are crazy expensive. All of this reflects my experience ordering outside of Bandcamp from small retailers and labels located in the same countries.
Amazon, CC Music and other large volume sellers like some on ebay offer cheap or sometimes free shipping, because they cut volume deals with shipping companies. I don't know of any small retailers who can compete with the low shipping rates offered by the big boys, but some offer free or discounted shipping for large orders as a sweetener, lots of Discogs sellers do this. But then, I rarely buy one LP at a time and if a Bandcamp seller doesn't have more than one item of interest, I'll look for it elsewhere.