I've never used them before, can you use an American credit card to order on this site? Do they convert automatically into Canadian dollars?

I've used them several times without any problems.

Yes, simply enter your credit card as you would on any U.S. site - the charge will be posted to your card in U.S. dollars (the conversion is done by your credit card issuer).

i tried ordering some dvds this past weekend off of amazon.ca

no problems putting them into the shopping cart, but i wasn't able to go to check out.

anyone else have this problem??

I had a similar problem last with with amazon.ca, i have multiple cc's on file with several of amazons sites, but i went to checkout and it said something about my cc being expired (it clearly wasnt, it lists the exp date right next to the card number). Only way i got around it was by adding a new card to my account.

Is your problem like the one mentioned in this thread about Amazon.com a few months ago?