Marketpress - add custom input fields to checkout page

Hi!
My client needs some custom fields added to the checkout process that they require from everyone who buys a certain item on their sites.

Any tips on how would I go about adding these to Marketpress? Is there a way to add them for only certain products or certain categories? If not, then I can just add them as optional fields to the general checkout page.

Thanks in advance for any help you can give me!