Fractional quantities or custom quantities by product category

// Define steps by category
function ccom_get_step_by_product_category( $product_id ) {

	// Categories in priority order
	if( has_term( 'wheels', 'product_cat', $product_id ) ) { return 2; }
	if( has_term( 'tires', 'product_cat', $product_id ) ) { return 2; }
	if( has_term( 'fabrics', 'product_cat', $product_id ) ) { return 0.25; }

	// Default to one
	return 1;
}

// Update min value to the quantity field
add_filter( 'woocommerce_quantity_input_min', function( $default, $product ) {
	$product_id = $product->get_id();
	return ccom_get_step_by_product_category( $product_id );
}, 10, 2 );

// Update step value to the quantity field
add_filter( 'woocommerce_quantity_input_step', function( $default, $product ) {
	$product_id = $product->get_id();
	return ccom_get_step_by_product_category( $product_id );
}, 10, 2 );

// Replace the WooCommerce filter that is validating the quantity
remove_filter( 'woocommerce_stock_amount', 'intval' );
add_filter( 'woocommerce_stock_amount', 'floatval' );

// PayPal compatibility
add_filter( 'woocommerce_paypal_args', function( $args ) {
	$custom = json_decode( $args['custom'] );
	$order = wc_get_order( $custom->order_id );
	$order_items = $order->get_items();
	$counter = 1;
	while( isset( $args[ 'quantity_' . $counter] ) ) {
		foreach( $order_items as $order_item ) {
			$product = $order_item->get_product();
			if(
				$product->get_name() == $args['item_name_' . $counter]
				&& strpos( $order_item->get_quantity(), '.' )
			) {
				$args[ 'amount_' . $counter ]
					= number_format( $order_item->get_total(), 2, '.', '' );
				$args[ 'quantity_' . $counter ] = 1;
				$args[ 'item_name_' . $counter ] .= $order_item->get_quantity();
			}
		}
		$counter ++;
	}
	return $args;
}, 100 );