The most economical way to ship your bikes would be by sea freight. It is way cheaper than postal service, Fedex or by air. But that means you will need someone to pick it up at a port. Some freight companies would forward it to a private address but that will considerably rise the price.
I bought goods a couple of times in Thailand for resale in the US (handicraft, cloth etc) and I used a freight company, the contact name was Habib and his mobile is (661) 7622919 (I'm pretty sure he does Europe too). I used him because at that time (3 years ago) he was one of the cheapest in BKK. I don't know if he's still in business.
Otherwise you can try them:
Flash Internation Transport Sea Freightflash@flashinter.com
Flash International Transport Co., Ltd.
149 Galaxy Place Building, 4th Floor,
Room No. 4/2, Nonsee Road,
Bangkok 10120, Thailand
Tel. 66-2681-2400 to 11 (Auto)
Fax. 66-2681-2413 to 4