…
Apparently this offer, even though clearly labelled “You're upgrading <MY NUMBER>" only applies to getting an additional SIM.
The help person asks their manager who “agreed to match this for you as an exclusive offer”… like, OF COURSE YOU DID, you don't get kudos for literally giving me a plan that your website offered me and is in my basket awaiting checkout and I'm trying to make it cheaper and easier to do by not sending a SIM and doing a number port.
Sheesh!!