They provide the limits they do for a reason. To allow not only evaluation but for one-time users to be able to get a small job done. Perhaps at a later time you'll be able to recommend it to someone who will become a customer or possibly find a true need for it in your own workflow.

So not only is it ethical, it's what the developers are expecting from people in your situation IMO.

Now you can do them a favor and mention the name of the program and provide a link, that could possibly be worth a couple of sales down the road.