We can't see the full set of "lower denomination" stamps on the letter, but I'm not 100% sure it's actually lower denomination. The sender of the stamps seems to be using the "2 domestic forevers + some amount of cents = 1 global forever" formula. I think the UK sender didn't need to include _two_ global forevers.
From the blog, the letter from California was dated April 2022, at which point the rates were domestic = $0.58 and global = $1.30. So the California sender correctly attached two domestics valued at $1.16 total plus an additional $0.14 to make $1.30.
It would be hard to know that ahead of time though. The global forever stamp is good for letters up to 1oz which can be as little as 4 US letter pages. It took the FSF 5 double-sided pages. Granted, it looks like lightweight paper & the post office doesn't seem to be very picky about this. But I think sending two forever stamps was being on the safe side.
>...... "Oh Well."
May have been more apt.
Is eBay really anyones first thought when looking for a (non-collector) stamp to (actually) mail?
Perhaps he should have picked up a few £1 coins on eBay, use them to purchase some stamps from the post office?...
I guess the a more "digital native" way would have been to first check if the USPS supports some kind of downloadable/printable stamping method, like QR codes or pre-bought labels (which, according to come early comment, they do).