Bethany. Here's my four cents (which of course is worth double the going rate
Photography, graphic design, Web design, art, dry cleaning, lawyers, etc are all pay-for-service. You have a service you need help with, you pay for it. Pay-for-service generally takes on two general forms: pay per project and pay per time. I'm sure you know the difference but I'll explain the two for anyone else who might not be aware of the two.
Pay per project is like it sounds. The client and service provider agree upon a particular outcome or service and the client pays as a whole for that service. Take dry cleaning. You pay $1.30 for the
"project" of having a shirt laundered.
Pay per time is paying someone based on how much time they spend doing your bidding. Lawyers and doctors are probably the most notorious making hundreds of dollars an hour.
OK, with any project YOU (the service provided) need to figure out which is more beneficial to yourself and the client. I have almost a decade of Web development experience so I'll use that as my example. Take a small Web site. I can charge a flat fee, $X to build a site. Or I can charge $Y/hour. Most of the time charging a flat fee is more profitable WHEN YOU KNOW HOW MUCH TIME IT WILL TAKE YOU and can minimize the risk of spending more time than you're paid. So if I know it'll take me 100 hours to build a Web site and I charge $50/hour then I would make $5000 billing hourly. Or if I were pretty sure I could get it done in 100 hours I might charge $6000.
Getting back to your specific situation, here are a few options I see you can try:
1. Charge for the entire project - Come up with a figure ABOVE how much time it will take you billing at $75/hr. If you think you can come up with 3 designs in 3 hours then charge something like $400 for the "project". The "project" will include giving them 3 designs to choose from and a limited amount of tweaks or changes that are available for them to make to the chosen design.
2. Charge per design- tell them it will be $75 for each design you come up with (make sure you agree up front how many iterations or revisions you will allow as they will ALWAYS have changes or suggestions)
3. Charge by time - stick to your $75/hour rate and tell them you can make as many (or as few) idea mockups for them to choose from but they are paying you hourly for this privilege.
4. Combine the two - charge $300 for an initial two designs with the knowledge that any time you put in for changes after that will be billed hourly at $75/hour
OK, those are just some ideas. Good luck to you!