You can mix and match with the 3 for 2 offers, the system will discount the cheapest product, you must however enter 3 products into the checkout to receive the discount.
On the label which is usually located on the back or the base of the unit, the serial number follows the model number.
We cannot say exact times when these will be available as it depends on products being returned to us, we advise to regularly check the website.
Yes these can be washed and should not need replacing if cleaned regularly.
It normally takes less than an hour to clean an average 12' x 12' room.