Great news! Your monthly limit renews every month! Your usage and billing cycles are based on the date that you sign up, not on calendar months, so your usage resets each time you are billed. Check out when your account's usage resets by going to Account settings (found by clicking your name at the top of your Shoeboxed home screen) and selecting My usage from the left side of the page.
Also, don't worry if you send in too many documents. If you send us more than your monthly limit, we will still process those documents! However, you will be charged a small overage fee on a per document basis.