add a new field to a product upon adding to cart.

Can someone help me find a link to the marketpress documentation? I'm looking to create a custom field that is attached to product upon purchase?

I have created a special plugin that interacts with a db table that is separate from wordpress. Upon the purchase of product, I need this table to be updated with the information that I will pass through this custom field that I am hoping to create.

I would love to learn more about the marketpress api... any direction would be very helpful!

Thanks!