This might require some account specific information to address, so I've submitted a case to Box Support on your behalf. You should be receiving an email confirmation shortly.
This was the answer I received from community support and it worked! They added 10 gig:
Please send an email to billing@box.com and one our billing representatives will reach out to you to help look in this or call 0808 189 0504. You may also include the screenshot that you downloaded the Box app.