Monthly Credit is $21 of credit that is deposited to your Rocksbox account on the first day of every month, and will be automatically applied to purchases from your Rocksbox sets. This Credit will expire at the end of each month and does not roll over month to month. But you will receive another $21 of Monthly Credit at the beginning of the following month. Please note that Monthly Credit cannot be applied toward your monthly membership fee - only toward in-box purchases.