Logitech.com is your best bet, and it's much easier to get 20% off from them.
Buy any old Logitech item (like a mouse or gamepad.)
Register it at Logitech.com.
Logitech will send you a 20% off thank you code!
So as long as you don't overspend on your initial item, hey, there you go. Or you might even have a Logitech item in the house you never registered.