Everything that is under you. Shoes, mattress, bike, car, desk chair etc.

Cheaping out on the above will cost you more in hospital and physical therapy bills.

Toilet paper

Office chairs are important, but people tend to overlook the rest of their office ergonomics. If you work at a computer you should spend money on an adjustable keyboard tray and monitor arms. Then buy a good ergonomic keyboard and vertical mouse or trackball to go with them. My back and wrists used to get sore by the end of the day. That hasn’t happened since I upgraded my gear.

Disclaimer: my wife has a chronic neck injury.

Car! We’ve had Renaults for years, but last time we went for a BMW and the difference is night and day. My wife had to do lots of stretching exercises just to go and buy bread. In the BMW she can happily drive 5-6 hours with no (extra) discomfort.

Laptops that are designed to support Linux

Coffee beans for sure (gotta love freshly roasted specialty beans!), but also vegetables (organic & local are the best). Oh, and also headphones, for sure.

To me these are all totally worth it because the jump in quality is very noticeable. After a certain price point, though, the law of diminishing returns kicks in hard

I totally get why you might not but for me, quality underwear has been a great purchase. They last longer, breath better, stay put, and synthetic ones are washable in a sink in a pinch when traveling(I mean they all are but these dry really quick)

PC power supply.

Never, ever skimp on a power supply. Get one from a reputable brand and with a rating above what you may actually need.

A failed PSU can in principle wreak havoc on any other components in the system, many of which are far more expensive than the PSU itself.