I got this answer from LoicTheAztec which solves the problem with the message. But, when clicking the button, it does not scroll down to the customer details on the Checkout page, which is the idea.
Hers's the code from LoicTheAztec:
add_filter('woocommerce_add_message', 'change_cart_message', 10, 1 );
function change_cart_message( $message ) {
$subtotal = WC()->cart->subtotal;
$href = is_checkout() ? '#customer_details' : wc_get_checkout_url();
return sprintf( __("Your new order subtotal is: %s. %s"), wc_price($subtotal),
'<a class="button alt" href="'.$href.'">' . __("Ready to checkout?") . '</a>' );
}
Can I please get help with that? Here's the link that question.
To get the scroll to checkout "Customer details" section, you will need to replace the following line:
$href = is_checkout() ? '#customer_details' : wc_get_checkout_url();
by:
$href = is_checkout() ? '#customer_details' : wc_get_checkout_url() . '#customer_details';