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Postage costs were by weight rather than shape or size. As special recognition of the Empire (and possibly also the emerging Commonwealth), postage by sea mail anywhere there cost the same as within the UK.
In my mother's effects was an old Pocket Companion booklet which listed postal rates. Although the booklet was undated, it gave a list of presidents of the USA, the latest of whom was President Roosevelt. Since he was in office between 1933 and 1945, that dates the booklet to between these dates. As it was almost certainly bought by my grandfather who died in 1942, the booklet must have been produced between 1933 and 1942.
I have no idea if or how postage rates changed during these years, but offer the list for your interest. Click the thumbnail for a larger legible image.