Service based industries like graphic design or architecture tend to contract clients in one of two ways: fixed fee or hourly. I’ve worked at both (as an architect) here are my thoughts. 

Fixed fee: Puts the onus on the company to streamline their service and be efficient with their time. Motivation is to increase productivity to scale profit—do more with less, take home more profit. The risk is a runaway design-first ego that backslides progress. If something is outside scope, additional services need to be negotiated, which often times makes people uncomfortable (silly but true). 

Hourly:  Easy to manage, easy to track, but is only scalable by working more hours (more employees or more burnout). There’s no motivation to be more efficient with your time. Given a fixed hourly work week, profit is static. You can’t bill hours twice, so finishing a project early and starting a new one is the same as just working more hours on the first. 