Hook for submit?

Is there an action hook for the ‘buynow’ button? I don’t see it, but I’ve been known to look over the obvious…

In case there’s a better way, here’s what I’m trying to do:
I have a cart full of items. I want to process payment for them but I don’t want to create a transaction in my database until the user actually attempt to check out. So… I’m looking for a hook that I can use to fire a function that creates a transaction with the user’s cart items and awaits return from paypal (via an IPN notification) to update the transaction – success of failure.

I admit I’m having a little bit of “coder’s block” here. This seems like it should be easy, which makes me feel like I’m missing something. I don’t want to create transaction records too early; users can simply walk away and I’m left with a bunch of abandoned transaction records attached to abandoned carts. If I’ve missed something in the general ‘digital purchase’ workflow, appreciate any advice from others. If not, and there’s a hook for the ‘buynow’ button, thanks!