Is there a way to add a custom item to Marketpress cart?

Hi There,
I have developed a plugin that allows users to ask a question to the site admin, they have to pay for their question and the admin can limit the number of questions asked per day. Currently, when the user clicks on "Ask Question" they are sent to paypal where they can pay for their question.

Ideally, I'd like to integrate what I have built with marketpress, so that when a users clicks on ask a question, it is added to the cart and they can go there and pay for it - this is because I'd like to take advantage of the coupon abilities in market press.

Is this possible? If so, do you have any guidance for me in terms of functions I could hook into to make this happen?