A travel charge recovers the cost of reaching work beyond the area a business ordinarily serves.
Why distant work costs more than it appears to
Drive time is paid labour producing no invoice, and it displaces work that could have been done in the same hours.
So a job an hour away at the same price carries a materially lower margin than one ten minutes away, and nothing in the pricing shows it — see tickets per route.
The two common structures
Zones. The service area divided into bands, each with its own charge. Simple to quote and simple for a customer to understand, and it works with how the business is actually laid out — see service zones.
Distance. A rate per mile beyond a stated radius. More precise and harder to quote quickly.
What the charge should cover
The paid travel time both ways, which is the larger part, plus the running cost of the vehicle — see vehicle costs.
Fuel alone understates it substantially, because the driver is the expensive part.
Absorbing it instead
The usual approach, and it produces a business where outlying work is unprofitable without anyone being able to identify why. The revenue is there and the margin is not.
Where absorbing it is deliberate
Building density in a new area, taken as a known cost with an intention behind it — see geographic expansion.
That is a decision. Absorbing it because raising it was awkward is not.
Grouping instead of charging
Where several customers exist in a distant area, scheduling them together removes most of the cost the charge exists to recover, which is generally better for everyone.