Maybe so, but you must be certain that in the end you make money on those 18 shirts.I agree with Jim55912 about the time spent with smaller 'walk-in' customers, but in the early months and years they will probably be a necessity.
I find that most of the requests that I get for very small orders are from folks that have already looked online. Some even have a design that they did on one of the big online t-shirt websites.
The largest seller of custom shirts online has a link to a quick price calculator at the bottom of their home page. I would recommend using this to figure your prices. Add 2.00 per shirt to whatever the site calculates.