The 20 planes are worth about $8.3 billion but list prices are often discounted for big buyers.
The order is a boost for the European plane maker because A380 sales have fallen short of expectations.
Amedeo CEO Mark Lapidus said the A380s will be delivered between 2016 and 2020.
A day earlier, low cost carrier VietJetAir signed a deal to buy more than 90 Airbus A320 jets worth about $9.1 billion at sticker prices.
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