The customer will never care that the transaction is delayed into the blockchain (delayed until the cartel's mining servers wins a block in the proof-of-work), because Amazon will give their customers 0-confirmation access to what they purchased.
So that delay is irrelevant.
Amazon won't care about it?
Sure, Amazon can trust its customers not to double-spend. But so can all the other merchants.
I already explained this upthread!
Customer would be banned from all cartel member sellers after one double-spend. Not very wise for the customer if the cartel is widespread in retail and online.
Also most purchases (not downloaded) ship after hours, so if the cartel has 15% of the network hashrate, any double-spend would be detected before shipping.