I used to be a very shy,socially awkward person. But i can promise you,retail changes that for the better. I was a waitress,then moved on to waterstone's,and am now at Greggs. I still get very anxious before work (ive always had bad anxiety issues which led to me becoming very introverted and shy),but when im there its actually fun and my shyness goes away. It is a serious confidence booster.
However,if you really wish to avoid it,then thats ok. It can be distressing. My dad is like you OP - shy, not a fan of customer interaction. Since 16,his career has gone :carpet fitter,postman,asda distribution,warehouse/forklift truck operative. You could consider these roles.