That's such a small thing tho. It was a tiny amount and presumably they would've refunded it had it gone through. It's also possible they would've discovered it themselves soon enough, since many companies only look at the money books at the end of the month.

Also, I doubt you'd get a "reasonable explanation" from any other company - at best they'll say "sorry it was a (human|software) error, we've taken steps to ensure it won't happen again", as disclosing anything more is usually against company policy.

Scammers use small changes on stolen cards as a test. If the test change doesn't get noticed or flagged, later a much larger is sent through.

"That's weird" is a not an acceptable answer for a card was charged for any amount.

Yeah, that was my first thought too, but the card was already expired, and a scammer would probably have known that immediately without testing (since they presumably need the expiry date as well as the card number to charge anything).

I've had mini-charges like these from other cloud providers, too, but then I could confirm the reason for it (some sort of storage I'd missed while deleting the instances).

the amount doesnt matter, it's about getting charged for no reason. You never know how much it would be for the next person they charge

It matters when evaluating their response. If they said "huh, weird" for a 200$ charge I'd be suspicious, but for such a small amount, I totally see why they wouldn't bother explaining how it happened.

you can't keep customers like that

I’ve used vast.ai many times over several years. I use the “top up” approach. Never any unsanctioned payments.

Key context here is vintermann had a balance due (and gave no indication that he disputes that). Of course there should have been a better explanation - especially if asked for. But to the extent there was a mistake, arguably it was simply in not charging the card earlier.