post office application process queries

As above, you need to confirm if you have a 0% purchase card or a 0% balance transfer (BT) card. Purchase cards are, as the name suggests, good for making purchases. Any spend is deducted from your available balance, but no interest is charged until the promotional period ends. 0% BT cards are used to transfer debt from one credit card to another, typically for a % fee of the balance transferred, and then interest is charged at 0% for the promotional period on the new card - the old card is paid off (but not closed). Either way 0% interest doesn't mean nothing to pay - you will need to adhere to the minimum payments for your card otherwise you may lose the promotional offer and be charged full-rate interest on your entire balance for the entire period the card has been active. You will need to closely check the T&Cs of the card you've applied for to know what type of card you have, what rates apply and for how long.

My guess is that the minimum BT amount is only if you elect to actually do a BT - it's optional to do this, and from what it sounds like you don't intend to do this. What determines how much you can spend is your credit limit - you won't know this until you have applied for, and been accepted for the card.