With American Express, you get 1 year extended warranty but if you buy the phone on contract, they won't get you a new phone. The max you can get is the purchase price, which is the subsidized price. They can fulfill their obligation by giving you $200 if you buy a 16GB iP6s on contract.
And if you buy the phone on a DPP, then you don't get any coverage from your credit card since you only paid the tax by credit card.
Credit card protection is only valuable if you pay full price for the phone - not a DPP and not a subsidized price on contract.
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With American Express, if the price of the repair or the replacement exceeds the purchase price, you only get the purchase price. If you buy a subsidized phone on contract, that is only $200 for a 16GB iPhone 6s.
I am willing to bet it works that way with all of the credit cards.
And of course, if you buy a phone on a DPP, there is no coverage through your credit card.