The "buy and add-on" function broke some time ago. I think it is a deliberate strategy to push people towards buying a recurring plan.
The workaround is to buy the add-on via the "out of data" message you get when you try to browse via your device. The limitation of this is that you can't do it remotely via another internet connection - ie: you can't add data to your granny's (or child's) phone or mifi unit without being in the house with her.
I should say that I like EE because I manage a number of mifi data units for safety and secruity on a number of ad hoc community events through the year. We need very little data and the low cost, occasional use ad-ons are perfect for our low budgets. We couldn't afford to use monthly plans and other operators don't offer low cost ad hoc PAYG.
It's a pity that EE are making it so difficult to access these add-ons.
