I mean, the sort of person who literally has no issue with getting people to what they want whenever they want?
I'm starting to feel a bit used. I have a friend with whom we alternate school pick ups every Wednesday. She seems to resent doing it when it's her turn. She emailed to ask whose turn it was this week and when I emailed back to say it was hers, she sent, "Fucker" back. Then another one to say she was joking. 
She's always calling to ask me for favours, never for a chat or anything. She's either late back for the school run from her day out in London so could I pick up her son and daughter and bring them home? Or the latest was the au pair "dropped the baby on his head" and she doesn't want to leave him so could I just pick up her daughter as I'm picking up mine. The dropping of the baby happened just at the right time of school pick. She wasn't so worried that she was going to take him to the doctor, mind. She left him 40 mins later to pick up her son early too so she clearly just didn't fancy doing the school run twice in a short space of time. (Our daughters do half days).
I thought we were friends. I think from her attitude, she thinks I'm just one of her staff. I wish I'd not even begun any arrangement with her regarding pick ups because she clearly resents doing it when it's her turn and is really starting to take the p*ss imo. I only started doing it because she had a baby six months ago and I thought it would be nice to help out by taking her kids out to the zoo, to the park etc but almost every time I set off to pick up my daughter, the telephone rings and it's her wanting yet another favour and she'll ring and ring and ring until I answer.
She's nice, very good fun and entertaining but I think she's a taker-kind of person. Obviously, one likes to help friends out but when it becomes like this, when the other clearly doesn't understand the limits, it's not helping out anymore.
How do I get out of this without offence? I take her son to two after school activities per week too - along with my son - so it's going to be tricky. I've just been put off having much to do with her really and need to back off. How?