Presumably, they use an off-line transaction.
The reason your credit card has raised numbers is that, in the old days, before ubiquitous data connections, a card transactions used a machine like this to transfer an imprint of your card details onto a form, using carbon paper.
You would then sign the form and the merchant would send it off to the bank to get paid.
It wasn't possible to verify in advance that you had enough money in your account but the deal was that the bank would honour the transaction and then come after you with a big stick.
The point of my mentioning this is that, although we're now very used to the idea that the merchant talks to your bank while you make a card transaction, this wasn't the case even 20 years ago.
Today, it's still possible to make a card transaction without verifying in advance that the card has sufficient funds, so that you can still buy stuff on your card even if the merchant's phone line or internet is broken.
In fact, the last-case fall-back is to go back to the swipe machines and carbon paper.
aviation.stackexchange.com/users/946/David Richerby
It will either go through the airline's existing air-to-ground system (like the seat-back phones you pay $5/min for) or they've got a store-and-process-later arrangement with the card companies.
(Just because you can't use a phone doesn't mean the airline can't, they just follow certain conditions better)
aviation.stackexchange.com/users/1632/paul
What is it called?
Buying stuff on an airplane during flight is know is in-flight commerce (IFC) .
How it works?
Credit cards are swiped via wireless handhelds on aircraft but the transactions are processed when the aircraft gets on the ground.
Limitations
Because of this billing mechanism – which sometimes results in fraudulent transactions – there is a ceiling of the value of items that can be comfortably sold today.
Read more here and here .
aviation.stackexchange.com/users/65/Farhan
I worked with the credit card processing at one of the bigger airlines a few years ago.
We used handhelds that stored the info from the magnetic strip of the card.
The transactions were uploaded to our back office system and then to the payment service provider after the flight landed.
The only "hard" protection is the modulo 10 checksum of the credit card number ( http://en.wikipedia.org/wiki/Luhn_algorithm ).
The fact that the airline knows exactly who is on a flight also makes it much harder to get away with this type of fraud.
During my time at the company I don't think I heard of a single case of attempted credit card fraud
(that doesn't mean there weren't any).
I think theft is a much larger problem in on-board retail.
aviation.stackexchange.com/users/3334/Johan
Transactions are offline.
The data is sent to the payment gateway on connection between the POS and wifi/GSM network on the ground.
Fraud happens, a lot.
Airlines have dedicated departments to chase these failed payments with varied success.
One Dutch airline, once the card ceiling was crossed (e.g. upgrades for a family might cost over €1000 which might be the ceiling limit set) would have the captain make a call to have the card payment cleared.
You're welcome,Mr J.
aviation.stackexchange.com/users/18083/Mr Johnson
If telephones/smartphones/tablets are not allowed on planes, how comes when you buy duty free items from the cabin crew and pay by cc they will use the card reader?  I suppose that in order to remove credit there must be some sort of connection to the various credit cards circuits in real time in order to achieve this?
How are credit cards processed during flight?
aviation.stackexchange.com
<security><cell-phone>
aviation.stackexchange.com/questions/8322
aviation.stackexchange.com/users/770/Fabrizio Mazzoni
Credit card transactions on airlines are done offline and then processed on landing. Alternatively, they may go through the airline's air-to-ground system.  The transaction either takes place via the airline's air-to-ground system or transactions take place offline and are processed once the aircraft is back on the ground.
Offline transactions are used. The transaction either takes place via the airline's air-to-ground system or transactions are processed once the aircraft is back on the ground.