SagePay integration in non WooCommerce website

I've received a request to implement SagePay gateway in WordPress website ecommerce using the theme: Traveltour ([login to view URL]) The theme has the following payment gateways: PayPal, Stripe, Paymill, Authorize, Hipay professional He needs to add SagePay as well, with option of refunds. He provided me the theme support answer: "You may check the include/stripe/[login to view URL] file ( in tourmaster plugin folder ) to see how we implement the stripe payment gateway and apply it to the one you want to use for this. You can search for the filter in the plugin to learn more about the function we use as well. Eg. add_filter('goodlayers_credit_card_payment_gateway_options', 'goodlayers_stripe_payment_gateway_options'); if( !function_exists('goodlayers_stripe_payment_gateway_options') ){ function goodlayers_stripe_payment_gateway_options( $options ){ $options['stripe'] = esc_html__('Stripe', 'tourmaster'); return $options; } } This function create the stripe tab at the tourmaster plugin option. If you search for the word goodlayers_credit_card_payment_gateway_options You'll see that it's called in include/[login to view URL] file. You can provide tourmaster plugin source code in: wp-content/plugins/tourmaster directory." In addition to this he provided the SagePay documentation: [login to view URL] I hope I've provided much detail as possibile. Could you please quote this request? Thanks in advance, have a nice day, best regards