In any contract it's a good thing to include some costs associated with Engineering Changes. Make sure that in the initial contract and prior to its signature, you define each of the following and your right to add these charges.

First, charge for each Engineering Change Request (ECR). This item makes it clear that your involvement to discuss changes to the original specification is going to cost the customer - and why not? If we assume a person has already been awarded a contract, time away from the implementation phase to discuss a potential change is jeopardizing the schedule and costing the implemeter the time he would have to used to implement.

Second, charge for each Engineering Change Order (ECO). This item makes sure the specification is changed (and signed by the customer in an addendum to the contract), and the implementer has had ample opportunity to assess the impact on his time, the project schedule and the material needed.