MyBitcoin has an SCI shopping cart interface but I don't know if they have the ability to send bitcoins instantly with an API.
For example, here is CampBX's instant payment call:
An API call to send Bitcoins to an address can be made in following format:
https://CampBX.com/api/sendbtc.php POST: user=USERNAME pass=PASSWORD BTCTo=ADDRESS BTCAmt=DECIMAL
What is the best way to send bitcoins instantly without having to wait for a block? I don't mind if two users have to have an account at Mt. Gox or mybitcoin.com to send bitcoin payments to each other.
You can send bitcoins to a Mt.Gox address, it'll be credited instantly (or you can make a Mt.Gox redeemable code, it works too, for BTC and USD).
We do not allow sending directly USD anymore (since June 27th) as it falls under "money transmission" and would require us to register to a crapload of things. If needed we may allow sending BTC instantly to a MTGOX account by number, however.