If Coinbase can cancel your order if the price goes up, but you can't cancel an order if the price goes down, then what they are doing is essentially stealing a free put option from you.  It is also disturbing that they destroy the evidence in the transaction history.  They should at least mark a canceled order as canceled rather than try to hide the fact that it ever existed.  As they say, the coverup is worse than the crime.

I have been having some similar problems with Coinbase lately too with no response from their support department.  It is very frustrating but nothing on the scale of problems people have had with other services in the recent past (MyBitcoin, Bitcoinica, Bitfloor, etc.)  At the very least, they should send an auto-response acknowledging that your request has been received.

In reality, I think these guys are just in way over their heads.  Hopefully they haven't been hacked or robbed and still have enough venture capital to pull their way out of it.  If so, I could see it being a great service at some point.  I will certainly make no attempt to continue using their service myself until it's very clear they're on solid footing, and I would definitely not recommend them to anyone, but maybe some day they'll be worth another try.