Firstly, good job on deciding to get one. I’m not sure what the quality of a drugstore vibe will be but try to avoid any that don’t use standard batteries. The small ones sold by Durex etc tend to use watch batteries which will run out really fast and are expensive to replace.
As for being comfortable. They don’t give a shit, you need to remember that. Those poor people are selling stuff all day long for people to put in all kinds of weird places in their bodies. They don’t know you, they don’t care. Just buy it.
If you want to turn it into something positive, edge yourself hard before you go and see it as a task that once you get that vibe home you’re allowed an orgasm as your reward. Up the excitement by going without panties and maybe in a skirt or dress. Simple hot things to really make it an experience to remember.
Good luck, and enjoy the orgasm!