Ah, I see. In the US a common marketing offer for credit cards is that they let you make one charge that will be interest-free for 6 or 12 months. You can pay it off whenever and however you like, but if you don't pay the whole thing before the deadline you'll owe all of the interest for the whole period. They make money off of this two ways:
1) People forget to pay it off before the final date. They're not always clear about when that date is; you know exactly how long it's been since the charge, but you don't know for sure when your payment will be applied. It's best to pay the total off at least a month before the final date, to be sure it's paid during the billing cycle before it is due.
2) If you use the card for anything else, there's fine-print that says that any payments will go towards non-promotional charges first. It can get very difficult to determine if you've paid the promotional charge off or not, and they typically do not assist you in figuring that out.
They also benefit from extending you more credit than you need for the one charge you want to make, especially since you're unlikely to use it in order to avoid trap #2. And they benefit from having an additional customer on the books. I'm sure these benefits make them money somehow too.
1) People forget to pay it off before the final date. They're not always clear about when that date is; you know exactly how long it's been since the charge, but you don't know for sure when your payment will be applied. It's best to pay the total off at least a month before the final date, to be sure it's paid during the billing cycle before it is due.
2) If you use the card for anything else, there's fine-print that says that any payments will go towards non-promotional charges first. It can get very difficult to determine if you've paid the promotional charge off or not, and they typically do not assist you in figuring that out.
They also benefit from extending you more credit than you need for the one charge you want to make, especially since you're unlikely to use it in order to avoid trap #2. And they benefit from having an additional customer on the books. I'm sure these benefits make them money somehow too.