This is one of the parts of prisons I find particularly distressing. I'm trying to find (but can't recall the name of the company) an article, perhaps posted here a few months ago, about a company that handles inmate "bank accounts". The percentage they took was absurd. Between what the financial company took and the prisons, $100 sent to a prisoner might end up only being $50 by the time they could spend it. Paired with the high prices in the commissary they can barely afford "essentials".
EDIT: JPay. Here's an article, but not the one I was looking for:
http://www.publicintegrity.org/2014/09/30/15761/prison-banke...