All the same as well, different versioning but equals same package you bought, updates often same dates.
Your license key is accepted on above 3 sites but not main site "A-PDF" where they also sell this program.
That's four websites for same program & likely more on the way.
FlipPDF equals FlippingBook v1.xx, it's an exact clone, they may have purchased sourcecode of 1.xx.
FlipPDF pro version..ehm, I wouldn't go there hence I think it's badly implemented.
Standard version is OK.. I have this version myself.
As for "Confusingly this website is an offshoot"
Wow.. then we should discuss video/ripping/burning type of software,
I lost count on some vendors (no names) but 10-15 offshoots are not unusual for one same program.
I don't like this strategy at all.