Best way to sell web development credits on website


I am creating a wordpress support website and I am confused about which plugin to use for a functionality.

I want to able to sell web development credits for members to buy with paypal.

The members will login to their members area and buy credits, 2,3,4 etc and then pay using paypal. Once paid they will be directed to custom page for filling up the form for the web development work.

There won't be any downloadable item as such.

So which plugin should I use with this? Would marketpress work for this or could I do it with something more simpler.

Also I have the membership plugin already installed, so I can have the members information filled out in the order pages automatically would be great.

Thanks in advance.