How did you pay for it, credit card? Request a refund of your money and if there's a problem with that then contact your CC company.
The members on this site have found some other companies with not so hot reps like Banana Hobbies as they have a very poor rating according to the BBB. They also use two other names but the sites and phone numbers are the same.
When I looked up info on this site about RC Planet I didn't see any bad feedback about them but that doesn't mean that others haven't had issues with them.
If they advertise the prices online and you buy it at that price, it's their loss. A hobby shop in Indiana had a major boo-boo on their site selling glow plugs incredibly cheap. They had to sell them at that price because of a typo.
[Update]
I just checked their site and it shows it as being in stock:
http://www.rcplanet.com/AquaCraft_U_...kasterisk-.htm
Here's something else that I just thought of. Most manufacturers like Aquacraft and Pro Boat have a set price that dealers can't go under to keep online stores and hobby shops even on their prices. Some offer discount coupons on purchases of items over a certain amount or throw in "free" extras to get around that. This site does neither.