Both shirts are changing next season, they are just trying to shift them.
For the home shirt, they can either reduce them by 50% to shift stock, as they've done with the away kit, losing about £20 revenue per shirt.
Or they can sell them at full price under this offer, getting an extra £20 for each one sold, and give away a seat which would otherwise be empty, costing them nothing.
It's actually a very sensible deal for the club.