I looked at using Bitrefill for buying some airtime for someone, thinking that with Bitrefill accepting LN then that should be the cheapest way for me to get a small amount (e.g., $10) to that person.

Well, the Bitrefill hidden fee (i.e., poor exchange rate) was about 10%.

So I sent $10 worth of bitcoin to the person directly, since on-chain fees are low right now, and they will (hopefully) just stack it instead.