Can a tier give a one off coupon when somebody reaches it, rather than a discount forever?

Yes. Each of the three tiers on Marketing → Loyalty & stamps gives one of three things: status only, a coupon code on reaching it, or a percentage off every order automatically. A coupon on arrival is a single reward at the moment they cross the threshold, and it costs you once. The percentage option is real money on every order for as long as they hold the tier, and the page says so in amber. Tiers are worked out from delivered orders each time, so a cancelled order never props somebody up a level. The Loyalty & rewards tab in the docs has a guide called Set up customer tiers.