The time in my life when I've ever felt most angry was not in a Carphone Warehouse, but it was for similar reasons to such upselling.
A relative pre-booked a hire car at Heathrow airport from one of the big rental companies. We picked them up at the airport, ferried them to the company's base (waiting for a bus would have taken forever), and stood with them to get their (pre-booked) hire car.
The guy there was the rudest guy I have ever met in my life (and I've met some really rude people, and would count myself among those people who people might rate as rude).
My girlfriend had booked and paid for the car online, because her relatives didn't speak English or have an English credit card. They all had EU driving licences. I was just the chauffeur to get them to the place so they could get the car and follow me down to Cornwall.
First the guy got into massive upsell mode. For everything. He took no hints, and constantly talked over us. We've paid, mate. Literally, we chose the car, know what we want, have a reservation, have paid for it, even typed in all the licence info. We'd just like the car.
Then he got beyond-shirty to almost aggressive. The beautiful thing was that he was supposed to be training an apprentice just behind him. The apprentice's face was an absolute picture for the entire interaction. I actually wondered whether he'd "bet" them that he could upsell us and was so frustrated when he couldn't. Literally the guy would ask about "Do you want to pay a deposit for the petrol usage and then you don't have to worry about returning the vehicle with a full tank?" The foreigners can't answer, the girlfriend can barely translate anything in time, and he's just not stopping talking to allow her to do so. So I say "No thanks, mate, we'd just like the car". He literally yelled at me.
Do we want extra-damage insurance (our paperwork from the website clearly shows we ticked no)? Do we want to set up an account? (for people visiting the UK for the very first time? No) Do we want to pay for a valet afterwards? And each time the guy doesn't want *me*, the only native English speaker, to speak - despite the fact that EVERY answer I give matches up with the eventual-translated answer. And he just doesn't understand "We'd just like the car, we have to be somewhere, and we've answered all these things for our booking already".
At one point, I leaned to my girlfriend and whispered "This guy..." and he asked me to leave. The apprentice's (and probably my own) face was utter disbelief at that point. Fortunately, my girlfriend had half a brain and said "If he leaves, we all leave, there's no way I'm not going to deal with you on our own." Nobody has ever infuriated me, before or since, to the point that my hands start shaking and my jaw clenches from the restraint not to give him an earful.
He got no extras. He got a complaint filed against him. My girlfriend and I knew that if we weren't absolutely reliant on getting a car that we'd already paid for, and quickly get down to Cornwall that same day, he'd have been on the receiving end of much worse than a bit of "We're not interested" and "No thanks".
Even the foreigners were kind of "What the hell is this guy's problem?" and they didn't even speak the language.
Honestly never been so infuriated in all my life. That one guy and his blinkered upsell cost that company more than he could ever make by doing so.