The Royal Mail in the UK is standard across the mainland UK, with shipping rates fixed within certain bands, up to a maximum, both inland and abroad.
These 'bands' cover letter post, 'small packet' and 'packet', the latter I think up to 3.5kg (?). Beyond that, parcel mail is used for large or heavy items.
The cost therefore depends on size and weight, or one of these (!).
For example, a package weighing, say one ounce, but measuring 8x10x4 inches, as an example, could cost the same as a package weighing 8 ounces, but only measuring 4x4x2 inches! The bands cover weight, and, since last year, width and length dimensions.
Bona fide businesses in the UK which ship lots, regularly, such as Hannant's, have a 'Business Rate', which of course works out less than a private individual would enjoy, and this rate is dependent on volume shipped per year - the higher the volume, the lower the rate - and this is passed on to the customer in the form of the 'true' shipping costs, normally at a fixed rate in the UK, and within size/weight bands for overseas.
Hannant's, for example, normally ship decals post free, in the UK at least, if ordered without other items.
So, bottom line when ordering from the UK - check the bona fide dealers, such as Hannant's first, before committing to 'private' sales, as the overall cost, including shipping, may work out less. Bear in mind that customers outside of the UK and EU countries, also have the (possible) benefit of not paying the Value Added Tax we pay here, which, at present, is 20%. (and whoever thought of that con of a title, replacing the old 'Purchase Tax', must have really studied trickery!!)