gistfile1.php

<?php
/*
 * This is the contents of $_SESSION
 * Array
 * (
 *     [qty] => 3
 *     [sizes] => Array
 *         (
 *             [0] => 10-x-12-x-1
 *             [1] => 10-x-12-x-1
 *             [2] => 10-x-12-x-1
 *         )
 * 
 *     [frequency] => 1-month
 *     [quality] => good
 * )
 *
 * There's only one product with a lot of variations. $this->product is the product itself
 * uses get_product( $id );
 */
public function shortcode_summary() {
 
 	if( !array_key_exists( 'qty', $_SESSION ) ||
		!array_key_exists( 'sizes', $_SESSION ) ||
		!array_key_exists( 'frequency', $_SESSION ) ||
		!array_key_exists( 'quality', $_SESSION ) ) {
		// At least one of them doesn't exist
		return;
	}

	// Let's localize the variables and clean session
	$qty = $_SESSION['qty'];
	$sizes = $_SESSION['sizes'];
	$frequency = $_SESSION['frequency'];
	$quality = $_SESSION['quality'];
	unset( $_SESSION['qty']);
	unset( $_SESSION['sizes']);
	unset( $_SESSION['frequency']);
	unset( $_SESSION['quality']);

	// check whether qty and the number of sizes is the same
	if( intval( $qty ) !== count( $sizes ) ) {
		return;
	}


	// let's get the products
	$variations = $this->product->get_available_variations();

	foreach ($sizes as $size) {
		foreach ($variations as $variation) {

			if( $variation['attributes']['attribute_pa_size'] === $size &&
				$variation['attributes']['attribute_pa_subscription-frequency'] === $frequency &&
				$variation['attributes']['attribute_pa_quality'] === $quality
				) {
				$variation_data = array(
					'attribute_pa_size' => $size,
					'attribute_pa_subscription-frequency' => $frequency,
					'attribute_pa_quality' => $quality
				);

				WC()->cart->add_to_cart( $this->product->id, 1, $variation['variation_id'], $variation_data );
				continue;
			}
		}
	}
}