andycronin

fixed by adding braces to the if statement,

add_filter('woocommerce_add_cart_item', 'add_donation_item');
function add_donation_item($item) {
	if ($item['data']->product_type == 'donation') {
		if ( (isset($_POST['donation_amount'])) && (is_numeric($_POST['donation_amount'])) && ($_POST['donation_amount'] > 0) )
			$item['donation_amount'] = $_POST['donation_amount']*1;
		else
			$item['donation_amount'] = 0;
		$item['data']->price = $item['donation_amount'];
	}
	
	return $item;
}

Example: Amount 5.00 entered on product page, donate button clicked, Item added to cart.

View cart, Amount is 0.00 (suggested amount from settings)