During your peak times, I would suggest extending your turnaround time. Right now you have a ship time of 3-5 days, and if that is too short of a time, then extend your time to 2 weeks. People would rather have quality than a quickly done project because you are "under the gun" so to speak, to ship their product. Most poeple know that custom work takes time, so extending the time shouldn't be an issue. Also, repeat customers are gold in a digital venue like this; it means that they REALLY like you stuff and want more! Don't discourage them by limiting how much they can buy! If anything, I would convo them and ask nicely if they don;t mind giving you some additional time to finish their custom work as you have a backlog of customers. I wouldn;t menthion that THEY are being put on the back burner because they are repeats, just ask nicely if they can wait a few days or a week.