Kiwi here, would never ever do that, I'd just say "a few". Amongst those few things there might be "a couple of apples" which would definitely mean exactly two, no more, no less.

I'd allow that "a couple of things" could mean 3, even 4 at a stretch - you're downplaying the size of the task to make it sound like your request isn't unreasonable. But a couple of some specific item (can of coke, etc.) means 2 - if they brought back 3 I'd ask them what's the 3rd one for.

Same here, in Canada; it's a metaphorical use, by my reading, similar to '<blank> will only take a second'