Follow up by OP:

Received the replacement battery from Gemini today, the same day I'm mailing the failed one back. Only took 3 business days to arrive via Canada/US Post. Great trust on their part, and nice of them to respond as quickly as they did.

So far, they're making all the right moves for a start-up: decent price, works as a MS replacement exactly with a little higher capacity, very responsive warranty service. If you can't wait for Geoman (it is winter and dark!) and want a no-hassle-no-effort replacement, I'd have to recommend it. Let's just hope my one failure of two batteries was only an anomaly - someone had to get the 0.01%-er, right?

And a shipping data point: (US) First Class International was $4; Priority Mail was going to be $21 (!); both would arrive in the 7-10 day window, neither provided any tracking. UPS and FedEx were ridiculous. I went First Class.