Add detection of the EDD shortcode cart.

functions.php

add_filter( 'tribe_tickets_plus_edd_cart_url', 'detect_edd_shortcode_cart');

/**
 * Get URL of EDD shortcode cart - if it is in use. 
 * If so use it for the tickets cart URL instead of checkout.
 *
 * @return string|null The url or null if the shortcode is not in use.
 */
function detect_edd_shortcode_cart() {
	global $wpdb;
	/** @var \Tribe__Tickets_Plus__Commerce__EDD__Cart $edd_cart */
	$edd_cart        = tribe( 'tickets-plus.commerce.edd.cart' );
	/** @var \Tribe__Tickets_Plus__Commerce__EDD__Main $ticket_provider */
	$ticket_provider = tribe( 'tickets-plus.commerce.edd' );

	// Look for a post with the shortcode.
	$query         = "SELECT ID, guid FROM {$wpdb->posts} WHERE post_content LIKE '%[download_cart]%' AND post_status       = 'publish' LIMIT 1";
	$results       = $wpdb->get_results($query);
	// Parse the results for the permalink.
	$shortcode_url = ( ! empty( $results[0]->guid ) ) ? get_permalink( $results[0]->ID ) : null;
	$cart_url      = empty( $shortcode_url ) ? $edd_cart->get_checkout_url() : $shortcode_url;

	// Add some query args for the cart/checkout/attendee registration checks.
	$cart_url = add_query_arg(
		[
			'eddtickets_process' => 1,
			'provider'           => $ticket_provider::ATTENDEE_OBJECT,
		],
		$cart_url
	);

	return $cart_url;
}