Many decades ago the notion of rush fees became taboo in some sign related industry segments and especially digital graphics. The era coincided with the popularity of services such as FedEx which offer finite service times and software being developed for industry at the time. Arbitrary fees could not be formulated, so the formula was, and still is, a service time option for the particular line item making it very clear to customers as well as fair to all parties.
Service today = $$$$$, service tomorrow = $$$$, service soon = $$$, service for savers = $$ is the concept.