MobiWork offers different turnkey services that you can use for your invoices, quotes, and work orders. One of these turnkey services, the "Service Call" type, allows you to charge a standard amount whenever a service call is scheduled:

This fee can be collected regardless of whether additional work is performed or it can be waived or discounted after the service is completed. Once the service is completed, users can apply a discount to the service fee to waive or reduce the amount.
If an invoice is generated from the work order, the "Service Call" line item will then be automatically added to it.