Upload attachments after Paypal payment?

I have built a site where users are able to submit content (attachments). But I want them to only be allowed to submit after they have made payment through Paypal. I am currently using the Contact7 plugin, but this obviously does not have this feature. I was thinking maybe direct them to a payment portal first, and then after the payment is complete is directs them to the contact form, but there’s not security keeping the users from directly accessing the site url for the contact form. Anyone have any good ideas?