No, there's no need to worry or cancel your order.
You can see the breakdown of promotions and rewards applied to your order by looking at the confirmation email. The total shown does not include any vouchers used.
For example, if the total shows £25.00 this doesn't include the £5.00 voucher. Therefore the amount actually charged would be £20.00 after the voucher has been applied.
The total can also be confirmed by checking the reserved and pending amount on your bank account.
In case you're still concerned that you have been overcharged you can cancel your order within 20 minutes from the order placement. Any vouchers used on a cancelled order will be available for use again within 24 hours of cancelling the order.