As far as I know, Royal Mail International Standard, which I take to be the equivalent of First Class International, does not restrict the "merchandise" contained by price, although, if it gets lost, you've probably had it. There are other tracked/signed options. I tend to add a customs declaration form (which can be printed from the Royal Mail website) to items to the US.
I have purchased over $20,000 of stamps, albums and printed literature from primarily GB and Western Europe over the last 10 years and I've never had any customs duty assessed.
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