WooCommerce Product Quantity Dropdown

WooCommerce by default adds a quantity input box to your product pages where customers can enter quantities, but in my case (I am selling wines) I need to have a dropdown with the options 1,2,3,6,12,18,24,30.

I have found this code written by GERHARD POTGIETER on his website. Thing is it only let me set the min, max and a step increment. I would have to have 3 steps? 1+1+1, 3+3 and 6+6+6+6+6?

Can someone help me out here?

<?php
// Place the following code in your theme's functions.php file
// override the quantity input with a dropdown
function woocommerce_quantity_input() {
    global $product;
	$defaults = array(
		'input_name'  	=> 'quantity',
		'input_value'  	=> '1',
		'max_value'  	=> apply_filters( 'woocommerce_quantity_input_max', '', $product ),
		'min_value'  	=> apply_filters( 'woocommerce_quantity_input_min', '', $product ),
		'step' 		=> apply_filters( 'woocommerce_quantity_input_step', '1', $product ),
		'style'		=> apply_filters( 'woocommerce_quantity_style', 'float:left; margin-right:10px;', $product )
	);
	if ( ! empty( $defaults['min_value'] ) )
		$min = $defaults['min_value'];
	else $min = 1;
	if ( ! empty( $defaults['max_value'] ) )
		$max = $defaults['max_value'];
	else $max = 20;
	if ( ! empty( $defaults['step'] ) )
		$step = $defaults['step'];
	else $step = 1;
	$options = '';
	for ( $count = $min; $count <= $max; $count = $count+$step ) {
		$options .= '<option value="' . $count . '">' . $count . '</option>';
	}
	echo '<div class="quantity_select" style="' . $defaults['style'] . '"><select name="' . esc_attr( $defaults['input_name'] ) . '" title="' . _x( 'Qty', 'Product quantity input tooltip', 'woocommerce' ) . '" class="qty">' . $options . '</select></div>';
}
?>