Using the web donation form, if someone navigates away from the page without completing the donation, then goes back to the main donation page and chooses an amount to donate, that new amount gets added to the previous amount. If the user wants to empty the cart, they're taken to a page with no navigation back to the main donation page.
The user should be able to delete individual items from the donation "cart" instead of having the empty the whole cart, which leaves them with no way to go back and donate again, unless they use the browser back button, which is what causes the multiple items in the first place.
Guest
