Shop is redirecting to a 404 page after adding ...

Go to Appearance > Editor and click on the shop.cart.php file

Look for this following code on about line 142:

if(isset($_REQUEST['simpleshop_checkout_go'])){
    header("Location: "._SIMPLESHOP_BASE._SIMPLESHOP_URL . ( (strpos(_SIMPLESHOP_URL,'?')===false) ? '?' : '&' ) . "simpleshop_checkout");
}else{
    header("Location: "._SIMPLESHOP_BASE._SIMPLESHOP_URL . ( (strpos(_SIMPLESHOP_URL,'?')===false) ? '?' : '&' ) . "simpleshop_redir=true");
}

change it to this code:

if(isset($_REQUEST['simpleshop_checkout_go'])){
    header("Location: "._SIMPLESHOP_BASE. ( (strpos(_SIMPLESHOP_BASE,'?')===false) ? '?' : '&' ) . "shopping_cart&simpleshop_checkout");
}else{
    header("Location: "._SIMPLESHOP_BASE. ( (strpos(_SIMPLESHOP_BASE,'?')===false) ? '?' : '&' ) . "shopping_cart&simpleshop_redir=true");
}