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.