Hi,
Add the code below to your custom css option to fix the related products width.
@media only screen and (max-width: 767px){
body.woocommerce .has-no-sidebar ul.products li.product, body.woocommerce .has-no-sidebar .upsells.products ul.products li.product {
width: 48%!important;
}
}
Can you explain further the checkout process issue? Maybe a screenshot would help me understand.
Thanks
-Rui