> 2. Large jar labelled "TIPS", partially filled with money.

That's my preferred one.

> 1. Point of sale system that explicitly prompts for a tip.

Seemingly the same general idea, but also for...

Buying things, as opposed to eating/drinking? (I've seen some at what was arguably grocery stores that also had some sit-down dining, paid on the same terminal.)

Bars that already include a 20% service charge, and the prompt says 26%/28%/30% on top of the service charge? (Fortunately, I've only seen that one once, so far.)

> 3. Check to sign with a tip line.

These can be confusing at takeout places!

Only very recently, after settling on leaving a "compromise tip" between nothing and a sit-down rate (and feeling appropriately mediocre about it) at a local takeout place, I ordered with the owner for the first time – and he immediately threw away the receipt as it came out of the machine with a smile, not giving me a chance to fill the tip line. Apparently no take-out tips expected!

I prefer the POS to the tip jar, because I tend not to carry cash.

If you're paying a service charge, you're not expected to tip. (I know, this is annoying; the most annoying thing here though is the service charge).

Oh, I do tip at the POS almost exclusively!

Still, a physical tip jar tells me "a person actually working here put this here intentionally", while a POS sometimes (probably irrationally) makes me wonder if these were just the defaults Square or competition shipped the thing with, an A/B test on proposed amounts run by a faceless corporate board etc.

I should have probably clarified whether we were talking about to-go orders, because obviously you would tip at a sit-down place, similar to a sit-down restaurant.

For to-go orders, 1. and 3. are often because they use the same PoS (usually a Square or Clover terminal these days) for the to-go and sit-down orders. You are free to read accidents of technological convenience as social cues, but that doesn't automatically make them so.

2. is probably a true social cue, for either sit-down or to-go.

I tip to-go coffee shop orders (when there are clear tipping norms, as is the case at all the coffee shops I go to). You can call the POS system an "accident of technological convenience", but it is also a prominent, clearly legible sign expressing the expectation that you're going to tip.

It is less of a big deal to stiff a coffee shop than a restaurant (you might actually get yelled at at a restaurant!) but if you talk to them when the shop is quiet they'll mutter under their breath about the regulars who don't tip.