What Operating System Can I Use With My Version Of Checkout?
Checkout 4 is compatible with Mac OS X 10.6 (Snow Leopard) - 10.11 (El Capitan)
Checkout 3.6 is compatible with Mac OS X 10.6 (Snow Leopard) -10.9 (Mavericks)
Checkout 3.5.7 is compatible with Mac OS X 10.5 (Leopard) -10.8 (Mountain Lion)
If you are considering upgrading your Mac's operating system to something later then 10.9 - you would need to upgrade to Checkout version 4. This is a paid upgrade from Checkout version 3.