You need to talk to her. Privately, and talk to your manager. Tell her that 1) You will not be belittled in front of clients. You were working with a customer, and her snatching the pen was rude, uncalled for and very unprofessional. You are capable of taking orders and writing down measurements. If she needs to read the form, ask her if she would prefer you fill out differently for work she will be doing.

About the sizing- tell her that you have your own work to do as well as assist her with her duties. You are happy to help her with sizing, but she needs to be very concious of your schedule- not just her own. If she needs you to be available, she needs to give you a full days notice (minimum) that she will be needing you the next day. What if you had a doctor's appointment or something you couldn't change? You are a co-worker, and again- happy to help. But you are not at her beck and call. You have your own duties you need to fulfill and your own schedule you need to keep. If she needs your assistance, she needs to let you know it's coming plenty of time in advance.

Then talk to your boss- or talk to your boss first and have the conversation in front of them. Make sure your boss knows you are a team player, and want to help. You DID stay because the company would have lost money if you hadn't. But this was a foreseen issue, and you cannot always drop everything because your coworker didn't have the courtesy to inform you in advance.

So what is the issue? That she made you feel foolish in front of a customer? When she took the pen, you could have said, "That's ok, I've got it." and she would have been the one looking foolish. If you must stay over, then ask for extra pay.

She is only thinking of herself when she doesn't plan well in advance. Is she the manager...or just another co-worker. If she is the manager, then you should ask for the extra $$, if she is a co-worker, tell her that her poor planning isn't your issue and offer to stay an extra 15-30 minutes. Then go home.

Hope this helps.