Hi Peter,
First off let me say that I use both FSUIPC and WideFS, and have found them to be both stable and effective; both of which is very hard to find in the 'freeware' category,as well as in a lot of the commercial and 'payware' software out there. A professional job, well done.
As a software engineer myself, I am supportive of your desire to charge for your products. I think it is totally appropriate. Some will grouse about having to pay for something they've gotten for free, but that is just normal.
I am curious about how payment will be made for your products? From my perspective, both FSUIPC and WideFS are development centric products, and not targeted for the und-user of the products in which FSUIPC is a key component. FSUIPC and WideFS have no use for the pilot as a standlone package. This is loosly akin to the VB runtime, libraries for developing graphics/network access/etc., and the end user doesn't pay for these directly. The costs are embedded in the products in which they provide functionality. And the pricing models and practices for this are already well established in practice, and allows for quite a bit of flexibility in how to price products.
Consider the scenario where a pilot has purchased three products, all of which integrate FSUIPC. but each implement different functionality of FSUIPC. Would the pilot have to buy 3 different licenses, one for each product/fucntion set? Or if a pilot purchased one license that exposed all the functionality in FSUIPC, then would that cause problems for the vendors of the products that have chosen to expose a smaller percentage of FSUIPC in their products and perhaps wanted to charge an extra 'Pro' fee to expose that functionality. It can get rather complex in implementation.
I would encourage you to not focus on whether to charge, but on how to charge.