After being completely offline for a week or so, the Bell Travel site (with buses from Hua Hin to Suvarnabhumi and back) is now online again. But with a decided downgrade: it now costs 50 baht to book online (it was free before, you just paid the price of the ticket) and you can no longer select your seat.
Another reader posted the company's explanation that there had been computerized overbooking and other such problems, but I've been riding the bus for two years and never saw any evidence of this, either from or to Hua Hin. I suspect that's just a cover story to justify raising prices.
While the site was offline--I called when I was in Bangkok and was told I would have to go to the airport ticket counter to book a ticket for the following day! which was definitely not an option--I looked into train travel, and found that for those of us who used to use the airport bus to get from Bangkok to Hua Hin and vice versa (not actually flying into or out from the airport), the train, although slow, is a pleasant, cheap, and enjoyable alternative.
Sorry, Bell Travel, I was long a fan of yours--in fact, I posted an announcement of how great you were on the Thai Visa site a couple of years ago, and right after that the buses went from nearly empty to almost always full! don't know if there's a connection--but the fee for online booking is not rider-friendly. Please consider getting rid of this!
As to the trains, prices range from 152 baht (non-aircon but quite comfortable 2nd class seats) to over 600 baht for fancy air conditioned sleepers. There is also a super-bargain train (free to Thai citizens, about 44 baht for foreigners I think) that runs once a day between HH <—> BKK. Ask at the train station for details, as this one doesn't show up on the SRT web site (you can't book it until an hour ahead of time, which means there are always seats available--but it's a very slow train with hard 3rd-class benches, so bring a cushion and something to read)!