I had my tax bill to pay today which was over £500. I was amazed at how I just went to the post office with my debit card and paid it. I popped in my pin and didn't even have a number on the screen to show how much money I was parting with.
This made me realise how easy it can be to spend money without realizing it. I knew how much the bill was for and had made sure there was enough money in my bank account to cover the cost. However, it could be very easy to go to a department store or supermarket, pick up a handful of things and then not notice how much it will all cost. You could even pay without looking at the total and it could be so easy to max out your credit card or overdraft in just a few shopping trips.
I stay aware of prices and always know roughly what my spend will be when I get to a till. However, if you do not do this and it is more than you can afford, it can be embarrassing having to put things back or walk out without buying anything at all. It can be easier to pay it and bear the consequences. It is far better to therefore have a clear idea of how much you can afford and what the value is of what you are spending, so this will not happen to you.