Imagine getting sliced with a razor blade over and over, about a thousand times each second for as long as it takes to get the tattoo. If someone said it didn't hurt, they are lying to you, or got a tiny little tattoo with no shading.
I admit, mine didn't hurt so bad at first, while he was doing the lines. In fact, it feels kind of good in between lines, when they wipe away the ink and blood, and add vaseline... I guess it appeals to my sadistic side

But after about 6 hours when he was finishing up the shading, holy SHIT that shit hurt!
That said, you are just going to get ONE letter, and the shoulder is definitely less susceptible to pain than some body parts. So you'll be fine. They'll probably be done in half an hour.
After your first tattoo, you will crave the pain. Not a day goes by that I don't think of my next one.. I can't wait! Might be this winter, if I can save up for a trip and another huge painful session