1. On the "choose your own box" page, https://www.elliscreekfarm.co.nz/shop/meat-boxes/choose-your-own-box/ you can select items for your box.
2. Click in the check-box for an item.  An Option choice appears for you to choose the packet weight you want. You can also select the number of packets you want.  
e.g.


3. Once you have selected the items for your box, scroll almost to the bottom of the page where there is a button saying "Add to Cart". e.g.

4. If it has a blue message like the one above, it means you haven't chosen the weight option yet. So you'd have to go back to that and select one, then scroll back down to add to the cart.
e.g. now it is able to be clicked:
5. That should get you to your "Cart" where you can checkout.