I've worked in retail for most of my working life, including 10+ years as a business owner, and it didn't take a pandemic to make people act like cunts.
With retail, IME, you have 3 grades of customers, friendly; functional; fuckers
There is little the friendly or functional customers can do to erase the interaction with the fuckers from your daily life but for me the secret was learning to appreciate the good ones more.
If you reach a stage where all you can remember from the day are the one or two (or three or four!) who make your life hard then retail has defeated you and it's time for a break, it's a very hard mentality to come back from
I also take particular joy in taking money from dickheads who think they hold the balance of power, despite the fact I'm the one profiting from the interaction, it's a pretty way to think but really customer if you're as great as you think you are then why are you giving me your money 
Also if your work allows it the occasional eruption at a particularly dickheady dickhead can provide some relief, but I suppose that's a benefit of being a business owner