After-hours work is work performed outside the normal working day: evenings, weekends, holidays, and emergency call-outs.
It costs more to deliver
Labour is generally paid at a premium, and where the hours push a person past the overtime threshold the statutory rate applies whether or not the customer was charged for it — see wage and hour basics.
Suppliers are closed, so parts come from stock or at a premium. And the work is usually less efficient, being unplanned.
It is worth more to the customer
Urgency is precisely what they are buying. A customer calling at nine on a Sunday is not comparing prices; they are looking for somebody who will come.
Charging the ordinary rate does not win goodwill so much as train the customer that urgency is free, and it loses money on every occasion.
Structuring the price
A premium rate, or a higher minimum charge for the call-out, published on the rate card rather than decided in the moment.
Stated in advance it is a term. Decided on the phone at nine on a Sunday it is a negotiation, and the business negotiates badly at that hour.
The cost to the people
Being called out repeatedly is one of the more common reasons skilled staff leave, and it is frequently undercompensated relative to how much it disrupts them — see employee retention.
A rota that is predictable, and paid for the availability rather than only the call, holds up better than an informal arrangement where whoever answers goes.
Whether to offer it at all
A legitimate decision. Emergency work wins customers and it consumes the people who deliver it. Offering it at a price that genuinely covers both is the middle path; offering it grudgingly at ordinary rates is the worst of the options.