Change the location

Put the code below in you functions.php. I wanted to place it between the Order summary and the payment methods so I used the woocommerce_review_order_before_payment hook. If you would like it just before the payment button use woocommerce_review_order_before_submit.

Something to consider. Clicking the “Add Donation” button reloads the checkout page, clearing the payment fields. If move the donation to after the payment options and your client has entered their credit card information it will be cleared and they will need to reenter it. They will probably find this frustrating.

This is a great reference for all the WooCommerce hooks on the checkout page. Makes it easy to place the donation exactly where you would like it. https://businessbloomer.com/woocommerce-visual-hook-guide-checkout-page

//* Relocate donation on checkout page
add_action( 'woocommerce_before_checkout_form', 'wdgk_move_donation_on_checkout_page', 5 );
function wdgk_move_donation_on_checkout_page() {
	remove_action( 'woocommerce_before_checkout_form', 'wdgk_add_donation_on_checkout_page' );
	add_action( 'woocommerce_review_order_before_payment', 'wdgk_add_donation_on_checkout_page' );
}