Hello,
The problem with responsive is caused by custom css – https://prnt.sc/j6syba You can add this code in Theme Options > Styling > Custom css http://prntscr.com/j6szmx :
.woocommerce .woocommerce-checkout .col2-set .col-1, .woocommerce .woocommerce-checkout .col2-set .col-2 { width: 100%; }
Regards
