Malcolm, for my postcards/stamps/coins/tokens business I use a company called CollectInsure based in the US -- they cover most branches of collectibles and antiques including jewellery and glass. I'm covered for up to $200 on 1st class mail and I forget how many thousands for Registered mail.
Alternatively, for my personal collection, I use Hugh Wood (Canada) based in Toronto (if memory serves). Hugh Wood insures mainly coins/tokens, stamps, and related collectibles. I haven't had any personal experience with claims but people I've spoken with have assured me that they settle claims quickly and with no hassles.
Neither of them are inexpensive but, as has been said here many times, I figure them as a "cost of doing business" and the peace of mind is well worth the cost!
Glenda
Glenda