I am charging for some of my modules, because I convert these from pdfs and sometimes I have to check the OCR errors by hand for 100-400 pages and that is a lot of work. I am trying to charge a small amount on some of these modules.
Please note that I do not get very many donations as is, and so I am having to do this. When I start getting 4-5 donations each month of $5 or $10 then I will probably drop charging for these modules. But I do need to have income to pay for hosting and domain fees. Currently I am one other person are working these websites, and the other person donates $20/month so I have to pick up the slack.