Unlimited isn’t possible because of how time and limits works, right? Is there ever an opportunity to market as unlimited without it being false?

Unlimited can be used honestly when it's practically impossible to reach the limit. For example if you are offering unlimited drink refills at a restaurant that is fair advertising because it's really unlikely your whole supply of soda syrup will be drained.

In this case it's clearly not viable because someone could easily hit a limit for design requests where it's just not possible to fulfil them on the budget. ~$500 is nothing like a months worth of unlimited design requests even if you only make a few requests per day.