Can't Purchase Data Boost

[scrow9]

I am on the Verizon Plan Medium 3GB and only have about 1% data left.  I have tried purchasing additional data for this month several times using the "Data Boost" functionality but it never seemed to take which was very frustrating.  However, just a few minutes ago I found some information online that said I can't purchase a data boost on my plan so I have a couple of questions:

1)  On my current plan what happens if I exceed the data allowance for this month?

2)  How can I purchase additional data for just this month/billing cycle?  (I don't want to upgrade my entire plan)