You can cancel monthly. You have to cancel at the latest by the 4th of the month in order not to receive the following Lockenbox of the following month. On the 5th of the month the next payment is due. If you cancel on the 5th or later, you will still receive the next Lockenbox, but afterwards you will not receive another one.

Here is an example:

Case one: You cancelled your subscription latest by May 4th? Then your cancellation is already valid for June, so that you will not receive the June box. 

Case 2: You cancelled your subscription on May 5th or later? Then your cancellation is valid by July, so that you will receive the June box, but afterwards no other box anymore.