Add the code below to your header.tpl to display the shopping cart

WHMCS v7.1 Template Six
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Search for

<!-- Shopping Cart -->
<div class="pull-right nav">
    <a href="cart.php?a=view" class="quick-nav"><i class="fa fa-shopping-cart"></i> <span class="hidden-xs">{$LANG.viewcart} (</span><span id="cartItemCount">{$cartitemcount}</span><span class="hidden-xs">)</span></a>
</div>

Replace it with this code

<!-- Shopping Cart -->
<div class="pull-right nav">
    <a href="cart.php?a=view" class="quick-nav"><i class="fa fa-shopping-cart"></i> <span class="hidden-xs">{$LANG.viewcart} {$totalitems} item(s) - {$totalamount}</span></a>
</div>

WHMCS v7.2 and latest Template Six
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Search for

<li class="primary-action">
  <a href="{$WEB_ROOT}/cart.php?a=view" class="btn">
    {$LANG.viewcart}
  </a>
</li>

Replace it with this code

<li class="primary-action">
    <a href="{$WEB_ROOT}/cart.php?a=view">
        <span class="fas fa-shopping-cart"></span> {$totalitems} item(s) - {$totalamount}
    </a>
</li>