Hello,

I experience a mixed-content warning when using HTTPS on the checkout page. It is caused by the form's action url: http://wc_get_checkout_url.

I have solved it by a small change in your code, file woocommerce/checkout/form-checkout.php:87. 

action="<?php echo str_replace('http:', '', esc_url( wc_get_checkout_url )); ?>"

I just got rid of the "http:" part of the string, so the "//" that is left makes it follow the protocol being in use.

I thought you should know about this problem. Maybe you can solve it in a more elegant way. If not, you can use my solution.