Products on a contract

I have made a service contract in OpenERP. Every hour my employee works on they contract is invoiced to the customer. That all works fine. But the work done, is sometimes in outr office and sometimes at the client site. When we work at the client site we have to add a transportation fee to the invoice. Is there a way, or have someone did some development, that i can add products on a contract/timesheet. Then i can add the transportation fee to the contract. Thanks!