Why can’t I purchase other items alongside my Core Pinter?

We have partnered with a separate fulfilment centre for our Core Pinters, meaning that it is not possible for us to fulfil orders with any additional items. Any orders that contain more than just a single Core Pinter will not be fulfilled.
My Bag ${ cartTotalItems } ${ cartTotalItems !== 1 ? 'items' : 'item' }
There isn't anything in your bag yet.
  Your order is in the bag
Sub-total ${ moneyFormat(cartData.total_price) }