Like a few other designers, I've added a 'donate' link on my latest skin.
To be honest, I don't expect anyone to donate for downloading a skin I've designed - I do them because I enjoy doing them, and people can download them for free.
However, there comes a time when you have to consider just how long you end up working on these things. I daren't add the hours up! And the more WindowBlinds is developed, the bigger the job of completing a skin becomes. I look on a donation as a personal thank you from the user for _all_ the skins I've done in the past, and I have 51 on WinCustomize at the moment. Although it won't mean that I start doing 'premium' skins (unless I'm specifically contracted to make one), it does help to keep the enthusiasm going.
Just as an example, my wife, who's hobby is needlecraft, just downloaded a pattern for some cross-stitch picture she wants to make, and it cost $10. The quality is good and the picture is nice - so it doesn't seem to be a lot to pay. Skins, on the other hand, don't have any other outlet than on the internet, so there is nowhere to compare prices.
So, the question is - is it bad to ask for donations?