It’s a tricky one. I’m sure you’d be able to find your groove if you stick to it. You’ll work out a routined conversation and you’ll end up saying stuff without thinking about it like ‘do you need a receipt?’ etc. I’m 22 and I find that most jobs require you to speak to people. It’s a pain but it’s something you sometimes need to do. Another thing to keep in mind is that, exposure might be really helpful for you. Talking to strangers all day will boost your confidence a lot. I say take the job. Try stick at it for a month at least and see how it goes