I had the same thing happen when I moved from waiting tables to doing phone support for a cable company. What saved me was picking up the phone more instead of always typing back, even when policy said chat was faster. Hearing someone's voice for thirty seconds tells you way more than a wall of text ever will. When a customer sends something short and scary like that, I started asking one simple question back, like "can you tell me what happened so I can fix it," and half the time they just needed to vent. The face part is real though, and I don't think that goes away. You just find little ways to bring it back, like a joke in the middle of a boring reply, and that got me through most days.

Watched this same thing happen at my store and what saved us was having a backup register ready to go before the manager even got there. I started ringing the coupons through one at a time and kept a running total on a sticky note so the lady could see what she was actually saving, which slowed her down but kept the system from locking up. The manager finally just did an override for the whole stack and told her next time it has to be one per visit. I also gave the folks waiting behind her a heads up that we would honor their coupons later that week, and most of them calmed right down once they knew they were not losing out. It still ate up a good chunk of my shift but at least the line kept moving instead of freezing solid.