Paid Support Ticket System

Hi all,
I need to build something and I'm here looking for advice.

Basically is a Ticket System where users have to pay to get support.

This can be done spending money for each ticket, or via a credit system (like cubepoints, or your Plugin "Upgrades") where users buy packages of credits and ther they make purchases using points.

In the beginning all tickets will cost the same, but in the future maybe tickets will have a different cost depending on a "category".
ex: if you need support on X it will cost 10, if you need support for Y it will cost 7.

I've looked a lot of Ticket Systems available online, both wordpress plugins than standalone solutions. But no one seems to feet my needs.
This because in all ticket systems you can open a ticket and WHILE writing it you choose the "category".
Instead I would make this choice before accessing to post screen, because it may have a different cost from another category.

Another important thing is that tickets must stay private: just administrators, authors and assigned moderators should be able to see it.

I need also both moderator and user able to attach files in a post.

Is there anyone that have ever managed something like this?
I would appreciate any kind of suggestion.

PS: I can even post this as a job request, if there's someone interested. (is there a place to post jobs?)