As I was reading your posts, a couple of things struck me. One is that if you are introverted and shy, that will be a huge barrier to a job in retail since a lot of that type of job is talking to the public and selling to them. I would suggest that this might not be the best fit for you.
On the other hand, a clerical job may be very well suited to someone who is shy. I don't really know why your height would be an issue for most clerical jobs unless it involved a lot of filing or carrying heavy things that they did not think you could manage. I think the advice to address questions head on is a good one. For example, if the job description says you must be able to carry 50lbs, then make sure they know that you can do that. A step stool sounds easy enough, but I'd still mention that you can work in a standard office with no more accommodation than a non-roling chair and a step stool if that's the case, since they may have visions of having to redesign the place to accommodate your height. One has to make reasonable accommodation or wording to that effect, so it will help to try and describe what that really means.
One issue that hasn't been mentioned may be how you present yourself. I think it is particularly important for you to present yourself professionally and confidently since that will send the message to judge you and not your height. It may be that interviewers are taking the combination of nervous, shy, and short stature and seeing a lack of confidence or seeing you as too young. Consider your appearance when you interview. Make sure that your clothes fit well and are professional in appearance. Some styles may work better than others for your body shape. It's not about being taller, but making sure you come across as confident, professional, and an asset to whatever business you are interviewing with.
If you are working with an agency, you might try asking them for feedback from your interview. It is unlikely someone would say they didn't hire you because of your height even if that was the reason, but the feedback may still be useful if they point out areas you could work on.
As an aside, I've seen a not particularly well trained man in a professional looking suit with every hair in place make a much better impression than a better trained and more experienced individual in an ill fitting suit with a bad haircut. The look shouldn't matter, but it still forms part of the whole picture.