I have someone posting a small item to me from the States, what is the cheapest/reliable method recorded delivery, the item is about spark plug size and would be about $15 from the UK including $50 insurance.

There is a U.S. Airmail Priority Post envelope that costs 9 USD and is limited to 2 lbs. in weight. It is a special mailing package and has padding.

It takes 10 days to get to Chiang Mai from the U.S. That was last year, don't know if rate has gone up since then.

is it recorded delivery and does i have insurance ?

I am sure you can have the parcel insured and registered, return receipt requested, at additional cost, but I don't know how much that would be. Certifying the parcel adds security but no signature required at delivery, but it is cheaper.

US Global Priority Mail has two flat-rate envelopes at about $6 and $9. Very reliable, I get them here in BKK in about 5 days. I believe you can insure them, you could find out by checking the USPS website.