When managing the assessment and repair/replacement of a faulty item, is there any expectation or obligation on the retailer to pay for postage?

I've had two situations this last few weeks where they've gone two different ways: the first, PB Tech was happy to send a pre-paid courier sticker for my dodgy Chromebook; they also paid for its return last time it needed repair (and I assume will do so when it gets sent back this time).

In the second, I have been billed by the dealer for the cost of them to send me a temporary replacement charging cable for our Leaf (something I'd not expected at all), and I assume I'll be charged when they send the repaired/replacement unit back later. I have also paid for sending our faulty unit back to them, and again expect to pay to send their temporary replacement back.

Any advice on how this is typically managed by retailers?