Donation doesn’t work

Hi,

I have the free version of Woo Donations plug-in so guests can add an extra donation on top of their product purchase in our online shop. The problem is when they click “View Cart” and it asks to “Submit extra donation here,” when they do that, it doesn’t add the donation to their total in the cart, not the added donation.

What am I doing wrong and why isn’t it working?

The page I need help with: [log in to see the link]