Products Page Customization

What do you mean by "single line" ?

You can add such CSS in the front end CSS file of HikaShop via the Display tab of the configuration:

div.hikashop_subcontainer_border {
border-right: 1px solid #CCC;
}

Please make sure that the "custom" front end css is select in that option of the configuration and flush your browser's cache.

Could you give a link to your website so that we can see that ?

Ah yes, that's because the border option of your listing is turned off. If you turn it on that should work.

Alternatively, you can use that CSS:
div.hikashop_subcontainer {
border-right: 1px solid #CCC;
}

That option is located in the hikashop options of the module with the id 101 used on that page to display the products listing. You can edit it via the menu Display->Content modules and you will have the "border" option that you can turn on.

If, you use that CSS, it will only apply to the listing of that page:

#hikashop_category_information_module_101 div.hikashop_subcontainer {
border-right: 1px solid #CCC;
}

You won't be able to apply the border on the middle column only with just CSS. You will have to edit the file "listing_div" of the view "product" via the menu Display->Views and change the line:
<div class="hikashop_product" style="width:<?php echo $width;?>%;">

to:
<div class="hikashop_product hikashop_product_column_<?php echo $current_column; ?>" style="width:<?php echo $width;?>%;">

so that you can use the CSS:
div.hikashop_subcontainer .hikashop_product_column_1 {
border-right: 1px solid #CCC;
border-left: 1px solid #CCC;
}