Something similar has happened to me:

My boss is out of town a lot and we don't have petty cash here. Whenever I need to mail stuff, I go and do it and save the reciept. Without saying anything to her, I leave the reciepts on her desk and post it saying "should I cut myself a check for this"?

You could try saving your ink cartridge reciepts and make a little spreadsheet for her adding in time, electricity and other costs. Kindly give it to her and say: I wouldn't bug you about this for a few sheets but since this is occurring so often I have to start charging you for this work. It's getting too pricey for me to do on a regular basis for free. I hope you understand.

EDIT*** I just read the above. This is not about a little Ink. This woman is clearly taking advantage of you. I would break it off immediately. She is just trying to make a buck off of you. Get away from this situation. AND she had no right to charge you for the referral. You don't have any written agreement so I'd tell he to go to hell.