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Wool socks if it's winter and/or you live in a cold climate.

Bidet attachment.

Fluffy stuffed animal. If I didn't have a couple stuffed kitties to hug I probably would have killed myself already. Nice to have something to pretend loves you

Silk pillow case. Good for skin and hair.

2kg of cookies

If you use your phone a lot (or if your battery is old and you haven't replaced it), a power bank is very useful.

A reputable powerbank (such as those from Anker) cost just under $20 USD for 10,000 MAH

A cheap over shoulder satchel, it's like having a extra pocket always available

Mini Swiss army knife. I don't know how to live life without this on my keychain

A beard trimmer. Shaving sucks and irritates my skin, plus I look like a baby without facial hair. But that's only one benefit.

I have been using a beard trimmer to cut my own hair for the last 10 years which has saved me probably $2000 worth of haircuts (estimating a $25 haircut + tip every 2 months). Not to mention saving about 1 hour waiting at the barber every time. I only messed up once and had to do a buzz cut.

Not luxury, but I keep a small AAA flashlight with me nearly all the time.

Easier to handle and faster to use than a phone light. Also doesn't use phone battery. I clip it on my hat if I need both hands like taking out the trash.

I have an Olight i3T but I would try out the i3T 2. More lumens and better battery efficency.

Gin.

A small USB-C hub. It has a USB-C out on one end then HDMI, 2 USB-A ports and another USB-C in on the other. I use it as my travel dock for my switch and steam deck, I've used it as a USB A to C converter, and I'll use it to test inputs on various TVs. I've gotten so much more mileage out of this $15 Amazon purchase than I ever would have expected

$20

A bamboo back scratching stick. I bought dozens back when Pier 1 existed and sold them for 75¢. Most are still around.

The Lady we got ours from isn't showing up at the farmer's marker anymore, but for well under $20, you can make your own corn pillow

This winter's been bad for deep freezes and 4 minutes in the microwave will keep my corn pillow hot for an hour or more.

Platinum Preppy fountain pen. Often, it's less than 10$

No one needs a fountain pen to write, but writing with a fountain pen is a unique experience. Alas, many fountain pens will cost a little fortune and other will cost a large fortune (what about a 40k$ one?). The Preppy is an exception. It's not the best fountain pen but it's very decent, it's even incredibly good considering its price. For slightly more 25-30$, I would also suggest the Lamy Safari.

Beside that, I can't think of many luxury items under 20. I mean, I can think of a few useful luxury things but they cost more than 20.

Freshly roasted coffee beans. A bag sets me back maybe $13 or so and gets me approximately 10 cups. Not what I would call outrageously expensive but I enjoy it a whole lot

I carry an arch lighter everywhere. It cost me $15 but saves me a ton in lighter fuel, and you'd be surprised by the number of every day situations that a controlled burst of plasma solves. Opening packages, lighting candles, kills mosquitos in the dark, and it's a fantastic taking piece as mine slots into a zippo.

Only downside is I can't use my zippo for light on the off chance I get trapped in a dark tunnel.

An electric 'coffee grinder' just for grinding up spices / seeds. Don't use the regular coffee grinder, or your coffee drinking housemates will get super annoyed. The flavor, and nutrition is so much better when you cook with freshly ground spices.