How to add custom price/fee in order total magento2

If you want to add custom price/custom fee in your total, then you can use following approach.

create sales.xml in you module etc folder

<?xml version="1.0"?>

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Sales:etc/sales.xsd">
    <section name="quote">
        <group name="totals">
            <item name="customfee" instance="Webkul\Test\Model\Total\Customfee" sort_order="150"/>
        </group>  
    </section>
</config>

Now, define app\code\Webkul\Test\Model\Total\Customfee.php, where you update your total according to your custom fee.

class Customfee extends \Magento\Quote\Model\Quote\Address\Total\AbstractTotal
{
   /**
     * Collect grand total address amount
     *
     * @param \Magento\Quote\Model\Quote $quote
     * @param \Magento\Quote\Api\Data\ShippingAssignmentInterface $shippingAssignment
     * @param \Magento\Quote\Model\Quote\Address\Total $total
     * @return $this
     */
    protected $quoteValidator = null; 

    public function __construct(\Magento\Quote\Model\QuoteValidator $quoteValidator)
    {
        $this->quoteValidator = $quoteValidator;
    }
  public function collect(
        \Magento\Quote\Model\Quote $quote,
        \Magento\Quote\Api\Data\ShippingAssignmentInterface $shippingAssignment,
        \Magento\Quote\Model\Quote\Address\Total $total
    ) {
        parent::collect($quote, $shippingAssignment, $total);


        $exist_amount = 0; //$quote->getCustomfee(); 
        $customfee = 100; //enter amount which you want to set
        $balance = $customfee - $exist_amount;//final amount

        $total->setTotalAmount('customfee', $balance);
        $total->setBaseTotalAmount('customfee', $balance);

        $total->setCustomfee($balance);
        $total->setBaseCustomfee($balance);

        $total->setGrandTotal($total->getGrandTotal() + $balance);
        $total->setBaseGrandTotal($total->getBaseGrandTotal() + $balance);


        return $this;
    } 

    protected function clearValues(Address\Total $total)
    {
        $total->setTotalAmount('subtotal', 0);
        $total->setBaseTotalAmount('subtotal', 0);
        $total->setTotalAmount('tax', 0);
        $total->setBaseTotalAmount('tax', 0);
        $total->setTotalAmount('discount_tax_compensation', 0);
        $total->setBaseTotalAmount('discount_tax_compensation', 0);
        $total->setTotalAmount('shipping_discount_tax_compensation', 0);
        $total->setBaseTotalAmount('shipping_discount_tax_compensation', 0);
        $total->setSubtotalInclTax(0);
        $total->setBaseSubtotalInclTax(0);
    }
    /**
     * @param \Magento\Quote\Model\Quote $quote
     * @param Address\Total $total
     * @return array|null
     */
    /**
     * Assign subtotal amount and label to address object
     *
     * @param \Magento\Quote\Model\Quote $quote
     * @param Address\Total $total
     * @return array
     */
    public function fetch(\Magento\Quote\Model\Quote $quote, \Magento\Quote\Model\Quote\Address\Total $total)
    {
        return [
            'code' => 'Customfee',
            'title' => 'Custom Fee',
            'value' => 100
        ];
    }

    /**
     * Get Subtotal label
     *
     * @return \Magento\Framework\Phrase
     */
    public function getLabel()
    {
        return __('Custom Fee');
    }
}