if you reached you cap fast like me(5 days) then you can message support and they will upgrade you to the next plan but they will deduct service days.

I had the flex $15 plan and i reached my cap at 5 days in so i msg support to add a boost of TB but all they did was upgrade my plan to the next level and left me with 15 days of service instead of 25 days. I got 1.75TB extra and instead of the 200 gig HD i got a 400 GB HD.

I was happy with that.