As you choose your badges, the badge order form shows you how much your badges will cost. When you've finished your order, it offers you a link back to here to make the payment.
If you have a credit card or a PayPal account, you can make an easy secure payment here. To start the process, fill in here the amount you need to pay - in UK pounds - and click the button for the next step. (Your money doesn't actually get taken till you end up on the PayPal site.)