EDD SL filter to add the checkout uri to the remote license check response.

edd-sl-filter_remote-license-check.php

<?php

/**
 * Add the renewal checkout URL to the remote license check response.
 * 
 * @param  array  $response   The response array.
 * @param  array  $atts       The atts for EDD_Software_Licensing::remote_license_check()
 * @param  string $license_id The license key.
 * @return array              The response array.
 */
function cn_remote_license_check_response( $response, $atts, $license_id ) {

	// Ensure the required functions exist; if not return $response.
	if ( ! function_exists( 'edd_sl_renewals_allowed' ) && 
		 ! function_exists( 'edd_software_licensing' ) &&
		 ! function_exists( 'edd_get_checkout_uri' ) ) {

		return $response;
	}

	// Ensure the license key exists; if not return $response.
	if ( ! isset( $atts['key'] ) || empty( $atts['key'] ) ) {

		return $response;
	}

	// If license renewl is not permitted; return $response.
	if ( ! edd_sl_renewals_allowed() ) {

		return $response;
	}

	// No need to add the checkout URI to the responcse if the license is not expired.
	if ( $response['license'] !== 'expired' ) {

		return $response;
	}

	// Bring into scope the EDD SL Class.
	$licensing = edd_software_licensing();

	// Get the download ID from the license key.
	$download_id = $licensing->get_download_by_license( $atts['key'] );

	// If the download ID was found, add the renewal URI to $response.
	if ( $download_id !== FALSE ) {

		$response['renewal_url'] = edd_get_checkout_uri( array( 'edd_license_key' => $atts['key'], 'download_id' => $download_id ) );
	}

	return $response;
}

add_filter( 'edd_remote_license_check_response', 'cn_remote_license_check_response', 10, 3 );