I thought this might be of interest to some:

Recently I sent a Honda Wave from Chiang Mai to Bangkok.

I had to take the motorcycle to the post office, together with 2 singed copies each of: The ownership page in the green book (I'm not the owner), the tax page in the green book, my passport.

The girl in the post office completed the relevant forms for me, I just signed them. Cost was about 1900 baht, it arrived 4 days later at the post office in Bangkok, where the recipient had to pick it up.

As it happens my girlfriend arranged the same thing today. Ubon to Phuket 3300 baht. Hope it gets there. 

I sent my scooter by post from Chiang Mai to Chiang Rai when I moved, IIRC 1200 baht.

Koh Lanta to Lop Buri - B3,000 for a PCX, post office to door step.

 

I took my Forza 350 from Bangkok to Surat Thani by train, Approx B750 for me in a 2nd class sleeper, and B1,150 for the accompanying bike. A tip of B50 each for 3 guys to load it, then I climbed onboard on Surat platform straight off the train and drove out of the station. Easy peasy...

Sent a CBR500R for less than 5k via Thai Post from Phuket to BKK. Delivery to the door (not post office). Bike was in BKK within 3 days but spent there 2 days before being delivered. Service at the post office was top notch. Prices for bikes over 400cc are higher.