If you don't OLCI, yes, you pay literally at check-in. A credit card, and you get a receipt on blue card, like a DM card or the last page of the former paper tickets. Wondered what would happen if I refused to pay. Would they refuse to issue me a BP, or downgrade me to Y?

I was told that people on C and F tickets have already paid the extra tax in the price of the ticket. It's only the upgraders they are going for.

No idea about pax UG'd at the gate, or OLCI. I shall try OLCI next time to see if that manages to avoid it. But they could still catch you at baggage drop-off, I suppose.