Use the navigation bar to go to Settings and then Messaging
Add a card if there is not one present. Select the size bundle you would like to purchase, select the card and click Buy Now
Confirm purchase. Once you have purchased, your invoice will appear on the right hand side where you can download it.
Your balance will appear at the top of the page. You can turn on the Auto Top Up toggle so that a new bundle will be automatically repurchased for you when you run out of credits to ensure there isn't an interruption to your service.