After your changes on November 7th (coupon on checkout), I faced many issues with the page and shop not loading properly. To fix this, I had to do a full backup of the site so that I have the version that was before you made the changes to the Checkout Page and Coupon. 

I lost 5 days of in searching and testing, but after restoring from the backup from the 7th of November, Lucky me - the website is working again. 

Now, the Coupon issue is here again - when entering the coupon on Chekout Page, it is blocking the payment section. You mentioned that only CSS was added (??), so please guide me on what CSS changes I can make by myself to ensure the coupon doesn't block the payment field.