the admin charge is less with royal mail than the couriers, so in my experience you are better off getting the supplier to ship using USPS if possible. That way, it is delivered by Royal Mail this end.
Anything over about $15 gets caught- I cant remember the last time anything slipped through from the USA. There are some funny rules if the import tax due is low, it is waived, but you will still have to pay VAT. Or visa versa.
On some items, it is still worth it- even if you get stung for shipping and import taxes. Normally, I try to find it in Europe first, but it isnt always possible.