install.xml

<?xml version="1.0" encoding="UTF-8"?>
<modification>
    <name>Bypass-Free-Checkout-Opencart</name>
    <version>1</version>
    <author>W Z | Bravo Net Solution</author>
    <code>Bypass-Free-Checkout-Opencart-Waziaq</code>

    <file name="catalog/model/extension/total/total.php">
        <operation error="skip">
            <search position="before"><![CDATA[$total_data[] = array(]]></search>
            <add><![CDATA[$this->session->data['carttotal'] = max(0, $total); ]]></add>
        </operation>
	</file>
    
    <file name="catalog/controller/checkout/payment_method.php">
        <operation error="skip">
            <search position="before" index="2"><![CDATA[foreach ($results as $result) {]]></search>
            <add>
            <![CDATA[if(isset($this->session->data['carttotal']) &&  $this->session->data['carttotal'] ==  0){
            foreach ($results as $key => $value) {if($value['code'] != "free_checkout") {unset($results[$key]);}}

            $this->session->data['payment_method']['code'] = 'free_checkout';} ]]>
            </add>
        </operation>
	</file>
    
</modification>