Build me a pro function on my website

I got a website where I let users sort stuff by categories and define languages in folders. It's a quite complicated website with many different variables which need to be taken into consideration before adding further content. As we have progressed I have realized that we need to open up space for a pro function on the website. This is where I want your help. Basically users should be able to pay using a payment processor as PayPal and automatically get the functions required.