For security reasons most online purchasers normally proxy through a merchant bank or similar service- ie their web site never takes the card details. The reques is handed over to the proxy site which takes the card details and amount to be deducted and does a few checks. If the sum to pay is less that a certain amount (£50 or so but depends on the card and arrangements) that the card has sort of agreement to pay out that sum. If its more that that then a credit check will be run and refused if insufficient credit on the card. Once the payment is 'agreed' normally the transaction is handed back to the vendor web site with a transaction ID. The money is not removed from the card at this point! In theory once the product is delivered the vendor company then the vendor can go to the proxy and get the money transfered using the transaction ID. Not many companies seem to wait till delivery to collect the money and of course you can rack up lots of agreed transactions before the money is removed from your account so the vendor is not guaranteed payment. Theres a lot more but I can here the delete keys going....
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