I'm glad this is taking off, I was worried I'd be making the whole thing myself :P

Ok, so I figure when you make a contract you just need to have a representative who can make that contract with you. It's probably easier to keep this part a little abstract. At low level a low level representative will likely show up but the amount of power they are willing to give you is related to how much they feel you can handle (read as your level). So in that way it would still be similar to other existing magic systems.

Also I figured there would be 3 primary types of contracts:
Indenture - basically a long term summon, they stick around and help you.

Service - a one time deal; probably something emulating a specific spell; short duration

Title - a permanent gain of some sort; higher price; these could be class abilities and you would only every get a few.

I pulled the names from and mimicked them after contract law.