Hi,
The problem comes from to things:
- You've activated the option "Use AJAX when possible for add to cart buttons" so that the system adds the product to the cart in ajax mode when possible.
- You've customized the product page layout so that the custom item fields are displayed after the add to cart button.
Normally, when the system sees that there are custom item fields to be displayed on the page, it deactivates the AJAX mode so that the add to cart button submits the whole form, including the custom field values.
In your case, that is not done and thus the product is added with AJAX, without the custom fields values.
Thus, the donation plugin doesn't see the value you entered in the amount field.
So either revert that modification, or deativate that option and it will work.