There's no obligation to buy every month. If you don't feel like shopping or being billed for a VIP Member Credit in a particular month, just log in to your account between the 1st and 5th to "Skip the Month" and you won't be charged. If you change your mind later in the month, you can come back to shop anytime. Whether you shop or skip, your VIP benefits will remain intact.
You can skip every month for as long as you want.