public function get_paypal_cart_api_url( $post_id ) {
		if ( empty( $post_id ) || headers_sent() ) {
			return home_url();
		}

		$cart_url      = $this->get_cart_url( '_cart' );
		$post          = get_post( $post_id );
		$post_url      = get_permalink( $post_id );
		$currency_code = trim( tribe_get_option( 'ticket-commerce-currency-code' ) );
		$email         = trim( tribe_get_option( 'ticket-paypal-email' ) );
		$notify_url    = tribe_get_option( 'ticket-paypal-notify-url', home_url() );

		$custom_args = [
			'user_id'       => get_current_user_id(),
			'tribe_handler' => 'tpp',
			'pid'           => $post->ID,
			'oo'            => [],
		];

		$invoice_number = $this->set_invoice_number();

		$custom_args['invoice'] = $invoice_number;

		/** @var Tribe__Tickets__Commerce__PayPal__Cart__Unmanaged $cart */
		$cart = tribe( 'tickets.commerce.paypal.cart' );
		$cart->set_id( $invoice_number );

		$items = $cart->get_items();

		if ( empty( $items ) ) {
			return '';
		}

		$product_ids = array_keys( $items );

		/**
		 * Filters the notify URL.
		 *
		 * The `notify_url` argument is an IPN only argument specifying the URL PayPal should
		 * use to POST the payment information.
		 *
		 * @since 4.7
		 *
		 * @see  Tribe__Tickets__Commerce__PayPal__Handler__IPN::check_response()
		 * @link https://developer.paypal.com/docs/classic/paypal-payments-standard/integration-guide/Appx_websitestandard_htmlvariables/
		 *
		 * @param string $notify_url
		 * @param WP_Post $post The post tickets are associated with.
		 * @param array $product_ids An array of ticket post IDs that are being added to the cart.
		 */
		$notify_url = apply_filters( 'tribe_tickets_commerce_paypal_notify_url', $notify_url, $post, $product_ids );
		$notify_url = trim( $notify_url );

		/**
		 * Filters the custom arguments that will be sent ot PayPal.
		 *
		 * @since 4.7
		 *
		 * @param array   $custom_args
		 * @param WP_Post $post        The post tickets are associated with.
		 * @param array   $product_ids An array of ticket post IDs that are being added to the cart.
		 */
		$custom_args = apply_filters( 'tribe_tickets_commerce_paypal_custom_args', $custom_args, $post, $product_ids );

		$args = [
			'cmd'           => '_cart',
			'business'      => urlencode( $email ),
			'bn'            => 'ModernTribe_SP',
			'notify_url'    => urlencode( $notify_url ),
			'shopping_url'  => urlencode( $post_url ),
			'return'        => $this->get_success_page_url( $invoice_number ),
			'currency_code' => $currency_code ?: 'USD',
			'custom'        => $custom_args,
			/*
			 * We're not sending an invoice anymore.
			 * It would mess up the cart cookies and we ended up not using it.
			 */
		];

		/** @var Tribe__Tickets__Commerce__PayPal__Main $paypal */
		$paypal = tribe( 'tickets.commerce.paypal' );

		$cart_items = [];

		$optout_key = $paypal->attendee_optout_key;

		foreach ( $items as $ticket_id => $item ) {
			$optout = false;

			if ( is_array( $item ) ) {
				$quantity = $item['quantity'];

				if ( ! empty( $item[ $optout_key ] ) ) {
					$optout = $item[ $optout_key ];
				}
			} else {
				$quantity = $item;
			}

			$ticket = $paypal->get_ticket( $post_id, $ticket_id );

			// Skip if the ticket in no longer in stock or is not sellable.
			if (
				! $ticket->is_in_stock()
				|| ! $ticket->date_in_range()
			) {
				continue;
			}

			$inventory    = $ticket->inventory();
			$is_unlimited = -1 === $inventory;

			// If the requested amount is greater than remaining, use remaining instead.
			if ( ! $is_unlimited && $inventory < $quantity ) {
				$quantity = $inventory;
			}

			// If the ticket doesn't have a quantity, skip it.
			if ( empty( $quantity ) ) {
				continue;
			}

			// @todo Figure out logic for storing optout for PP.

			$args['custom']['oo'][ 'ticket_' . $ticket->ticket_id ] = $optout;

			$cart_items[] = [
				'quantity'    => $quantity,
				'amount'      => $ticket->price,
				'item_number' => "{$post_id}:{$ticket_id}",
				'item_name'   => urlencode( wp_kses_decode_entities( $this->get_product_name( $ticket, $post ) ) ),
			];
		}

		// If there isn't a quantity at all, then there's nothing to purchase. Redirect with an error.
		if ( empty( $cart_items ) ) {
			/**
			 * @see Tribe__Tickets__Commerce__PayPal__Errors::error_code_to_message for error codes.
			 */
			return add_query_arg( [ 'tpp_error' => 103 ], $post_url );
		}

		$item_counter = 1;

		$has_multiple_items = 1 < count( $cart_items );

		// Whether we use add or upload depends on if we have multiple items.
		if ( $has_multiple_items ) {
			$args['upload'] = 1;
		} else {
			$args['add'] = 1;
		}

		foreach ( $cart_items as $cart_item ) {
			$arg_modifier = '';

			if ( $has_multiple_items ) {
				$arg_modifier = '_' . $item_counter;
			}

			$args[ 'quantity' . $arg_modifier ]    = $cart_item['quantity'];
			$args[ 'amount' . $arg_modifier ]      = $cart_item['amount'];
			$args[ 'item_number' . $arg_modifier ] = $cart_item['item_number'];
			$args[ 'item_name' . $arg_modifier ]   = $cart_item['item_name'];

			$item_counter ++;
		}

		$args['custom'] = Tribe__Tickets__Commerce__PayPal__Custom_Argument::encode( $args['custom'] );

		/**
		 * Filters the arguments passed to PayPal while adding items to the cart.
		 *
		 * @since 4.7
		 *
		 * @param array   $args
		 * @param array   $data POST data from buy now submission.
		 * @param WP_Post $post Post object that has tickets attached to it.
		 */
		$args = apply_filters( 'tribe_tickets_commerce_paypal_add_to_cart_args', $args, [], $post );

		$cart_url = add_query_arg( $args, $cart_url );

		/**
		 * To allow the Invoice cookie to apply we have to redirect to a page on the same domain
		 * first.
		 * The redirection is handled in the `Tribe__Tickets__Redirections::maybe_redirect` class
		 * on the `wp_loaded` action.
		 *
		 * @see Tribe__Tickets__Redirections::maybe_redirect
		 */
		$url_args = [
			'tribe_tickets_post_id'     => $post_id,
			'tribe_tickets_redirect_to' => rawurlencode( $cart_url ),
		];

		$url = add_query_arg( $url_args, home_url() );

		/**
		 * Filters the add to cart redirect.
		 *
		 * @since 4.9
		 *
		 * @param string $url
		 * @param string $cart_url
		 * @param array  $post_data
		 */
		$url = apply_filters( 'tribe_tickets_commerce_paypal_gateway_add_to_cart_redirect', $cart_url, $cart_url, [] );

		return $url;
	}