I'm an introvert (or was more at the time) and I once got a job in retail.
My first job was a cashier at a very busy, high-street shop that anyone would know when the name is mentioned. As an introvert, it was absolute hell. It literally required so much energy to act correctly and be able to communicate with hundreds and hundreds of customers, that I was so exhausted by the end of the day that I could barely walk.
And obviously I had a massive need for money at the time (I needed money for sixth form transport). But man, did I discover that I never EVER want to work in retail, ever again. I'd rather turn to drug dealing if I needed the little money you actually make from retail. I'm a bit better now (less of an introvert), but only due to years of evolution of my social skills.
So I guess the moral of the story is, if you know that you simply cannot handle it, just stay well away. It's good to try and not be an introvert, but somewhere along the line of being able to not act like an introvert, you will have some serious problems. For me it was a huge lack of physical and mental energy.
But I guess you should also be patient with it, so you can get used to it. It took me about 2 weeks, but I eventually adjusted to it. I call it personal evolution. Kind of like how the first homosapiens didn't have a clue how to make food, all they could do is scavenge in a harsh, animal world. But thousands of years later, we now have the luxury of sitting in our own private houses, stuffing our faces with Mcdonalds.