Let me take a stab in the dark: you've found that folks who pay less for your application have higher expectations for it, are unwilling to read the documentation, are ruder, and cause many more support requests per customer than your customers who paid full price do? How'd I do?
This phenomenon is very common with shareware developers. Higher prices tend to scare away many of the pathological customers.
I had no qualms today about raising the price of the iPhone app back up, now that it's fallen off the top 10 charts. It's kind of a support intensive piece of the app (e.g. it requires wifi to sync, and a lot of people have shitty wifi).
I did release an update right before MacHeist that fixed some edge cases in the syncing (see my presentation here: http://news.ycombinator.org/item?id=899656). That seemed to help somewhat.
I've found the difficulty of the support load can be better quantified by the number of unique requests rather than the number of requests.
I hired someone to help a few months ago to do support. I felt felt very lazy/extravagent/unresourceful doing so, but it's been a huge help, especially with MacHeist. She can't troubleshoot every issue, but that's perfectly fine, because she does a great job weeding out the riff raff. And she has a way thicker skin than I do dealing with customers.
I also launched a fancy new support section for my site at the same time as the MacHeist thing.
Also, there's a special box for MacHeist visitors at the top of every page that kinda tries to get them to buzz off:
I try to strike the balance between "I am always, always happy to talk to my customers" and "There are thousands of you and only one of me so I can't talk to you all every day". Mostly I fix the app and website to answer the common issues without requiring my input and, for the recurrent ones (cough Registration Key management) emphasize that they get their answer a day quicker if they use the lookup form than if they send me an email asking me to, essentially, please use the lookup form for them.
(Apropos of nothing: I would gladly abandon Registration Keys if people actually paid money for software that wasn't access controlled. Happily, the change from downloadable software to a web app has eliminated this issue for about 70% of my customers.)