Edit: found a link with some information https://www.uponarriving.com/amex-extended-warranty/#Warrant...
“The question gets a little tricky when you’re talking about Applecare. Amex should provide you with two additional years of coverage with Applecare since plans are usually 2 to 3 years. The issue is that some have reported they were denied coverage since Applecare was considered secondary coverage”
“If the item is also covered by a purchased service contract, the Extended Warranty Period begins at the end of the purchased service contract and extends the original manufacturer’s warranty for a period of time equal to the warranty, up to two (2) additional years.”
So yes I think Amex extends the warranty after (prepaid 2 year) AppleCare expires for a term of one additional year, taking coverage to 36 months.
In my specific case, they paid me on an iPhone at 23 months (when no AppleCare had been bought) and two MacBook Pros in months ~40 on which AppleCare had been bought.
So pay up front on the more expensive phone is $149 for 24 months or $7.99 monthly. 24 x $7.99 = $191.76 (42.76 more than up front). But what for the American Express customer, buying the phone. He or she gets the Apple 2 years plus 2 years from Amex so 48 months of coverage for the original $149 vs $383.52 paid via subscription (a $234.52 savings).
Someone keeping the phone the full 60 months it receives support from Apple would pay a total of $479.40 in AppleCare+ premiums. They did get an extra year of support from Apple vs that of the American Express customer paying up front at an effective aggregate cost of (479.4−149)/12 = $27.53 per month in the final year for coverage.
The American Express customer must of course also be saving for the replacement of his or her iPhone in 48 - 60 months. Presuming he or she wants to have the cash ready if the phone dies in month 49, they must be setting aside (999 × 1.08) ÷ 48 = $22.48/month for 48 months and then can coast for the final 12 months maybe earning a little interest before buying the new phone released in September 2024.