If your module is ready, you will see fee in checkout page like this picture: (WHMCS Default Theme : Six)
your_order_form = Current active order form.
You can add three variable in checkout page. (orderforms/your_order_form/viewcart.tpl)
g_total = Total product price + Payment Fee amount
Using :
<span id="totalDueToday" class="amt">{$total}</span>
replace to
<span id="totalDueToday" class="amt g_total">{$total}</span>
g_fees_text = Displays the Payment Gateway Fee Text in the module options. <li class="list-item" data-subtotal>
<span class="item-name "></span>
<span class="item-value"></span>
</li>
replace to
<li class="list-item" data-subtotal>
<span class="item-name g_fees_text"></span>
<span class="item-value g_fees"></span>
</li>
IMPORTANT: If you dont show not fee, you must add class “has_g_fees” in li tag. <li class="list-item has_g_fees" data-subtotal>
<span class="item-name g_fees_text"></span>
<span class="item-value g_fees"></span>
</li>
IMPORTANT2:
These css classes do not add anything to your theme. It is just the code required for JavaScript to add the value if that class exists.