As others have said, be sure of all of the details, not just part of them. Generally, the fsc is based on a formula calculated to make fuel cost $1.25 per gallon net. It is based usually on the national average price for fuel. It may vary somewhat depending on which company the van is leased to. Whoever pays for the fuel should get 100% of the fsc paid by the company. If you pay for all of the fuel you should get all of the fsc money to offset some of your fuel expense.
You also need to be aware of other expenses, occ/acc (occupational accident) insurance, qualcomm, tolls, parking, anything and everything that might have a fee associated with it. You need those details in writing also.