You can reserve items by selecting/swapping a subscription or by adding a one time product.
TO RESERVE AN ITEM AS A RECURRING SUBSCRIPTION:
After purchasing your first subscription you will be able to log in to your account and see your subscription. When you see the text "choose color/print" you can go ahead and select the product you want.
Click on manage.
Select the style, size and color you want and click on save changes.
You will now see a subscription with a selected color. This means your pair is reserved.
TO RESERVE AN ITEM AS A ONE TIME PRODUCT:
To reserve a one time product you can add them to your subscription by going to the page of the product and selecting "Add on time to your subscription" and clicking on ship with next subscription.
Once logged in you will see your one time product has been added to your dashboard. The item is now reserved!