I've seen mixed results on that. Some people have said it helped them to work a customer service-type of job, and others have said it didn't help or even made things worse. I think it depends on exactly what your issues and limiting thoughts/feelings are.
For me, customer service jobs don't help. The only thing that makes me relax is knowing what I'm doing. I have to know exactly how to work the cash register, exactly how to answer at least 95% of the questions/concerns a customer will bring to me, exactly where everything in the store is...and even then, the customer has to be friendly, and I'm still going to count the change at least twice (probably more) to make sure it's right and still worry later on about whether or not I gave out the right amount, did I put the receipt/money in the cash register like I was supposed to, etc. Without knowing everything and the customer being friendly, I will freak out.
I have had several jobs handling money and dealing with customers, and it's the same thing every job since about 2010. Last week, a woman gave me two 20s for a purchase that cost $26 and some change, and long after she left and it was done I freaked out trying to remember if I put the two 20s in the register or did I leave them on the counter and someone else got them/my co-workers saw them on the counter.
Of course, if you're talking about actual shyness and not SA or a combo of shyness/SA, then my response might not be totally correct for you...that might also be the reason for why some say customer service/cashier jobs help them and some say it doesn't, i.e. some are just shy and some aren't but have SA.