After adding the initial item to the cart, you can either return to the main page by clicking Online Store, or by clicking the category of the item selected, located in the top left-hand corner of the page. This will bring up the entire list of categories and you can select from that page.
What if the cart jumps around when I click Checkout?
Usually, this is solved by slightly shrinking the screen. If you cannot fully see the Checkout button, use either Control, or Command (for Mac), and roll back on the mouse wheel, or hold Control, or Command (for Mac), and the - (Minus) button. This will slightly alter the size of the screen and should fix the issue. For PC: Control+Roll or Control+-(Minus) For Mac: Command+Roll or Command+-(Minus)