There are various ways of delivering downloadable files from a website into the hard disks of an expectant public. Drupal (the content management system that the site is based on) has its own UberCart system which I did try our briefly, however, in the end I stuck with E-Junkie, the same system that I was using on the Flying Pig website.
I've been with E-Junkie for a couple of years now and, apart from the name, I've always been really happy. They are helpful, quick to respond to questions and very professional. I just hate the name!
Anyway, I'm rambling – my point was, I was looking through their documention when I came across the option to offer items for a variable price. In the admin options I have a couple of variables. Minimum price and suggested price. In the free version of the Reindeer I've set the minimum price to £0 and the recomended price to £0. When people check out they'll be able to pay more if they wish but won't be obliged too. In the next free model, probably the dog, I'll try setting the minimum to £0 but the recommended to £2. That way visitors to the website will have the option to get the model for free but will have to take positive steps to do so. Keep coming back – I'll let you know how it goes.