In order to use the built in SMS text messaging feature, you will need to purchase credits at 6p each. One credit can be used for a 160 character message. If a message exceeds 160 characters, further credits will be used (you can see a character and credit count as typing an SMS message).
There is a minimum purchase of £2.00 due to credit card fees and this minimum spend gets you about 34 credits (£2.04 for 34).
Type in the box how many credits you wish to purchase and the amount will be dynamically shown.
Hit purchase and the same card already registered for your subscription will be used to make the purchase.