I agree--so charge extra for fewer apps. Or make the weed control or fert an optional extra in September.
$50 for fert. $25 to add weed control. It costs you more--charge them more.
Also you can apply herbicide as a liquid with a skid sprayer, and add a small amount of fert. If they want to pay for only one trip over the lawn--fine--but you don't spend twice as much time.
Also consider "Lockup" a new herbicide which is applied as a granular--but it is highly effective. One trip over lawn.