The PV must be spent on one single order (it is not cumulative throughout the month) and can be on either a regular order or essential rewards order.  Log in to your virtual office for more info, or to place or modify your order.

Also, please note that you will not see the rewards added to your cart.  Once you reach the different rewards levels, they will be automatically added into your shipment, and you will see them pop up on your order (under recent orders) once it has processed.