Bad UX of quantity feature. In your case in order to increase the number of items in the cart, users have to click on the field --> write number --> scroll down to update cart button --> click on it. These are 4 absolutely unnecessary actions.
Instead, implement the quantity feature with plus and minus buttons and update the cart automatically after a click on it.
The Donation section takes so much space on the cart page (around 40% of the screen), while it can be implemented in just 3 lines: title, description, horizontal checkboxes
Improve the design of the Donation section by placing the checkboxes horizontally in one line with just the checkbox + amount of money for donation. As a result, your cart will look less cluttered and users will not have to scroll a lot to reach the Checkout button.
You have quite good shipping conditions as it's free on orders over $50. But you don't have a prominent notification about that on the cart page to motivate users to spend more to get free shipping. The current top notification is not visible when users scroll further down the page, as a result, many of them will just miss it.
Consider using sticky top notification about free shipping on orders over $50 that will be always visible to motivate users to spend more money.
I hope these recommendations will be useful to you. Feel free to ask any questions.
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