They presumably assume they’d be selling so little that it wouldn’t be worth the trouble.

They’ll probably wait out this situation for a while and see what the competition does…

If you just do it on your own computer, the packet will be already dropped by your own gateway. You can fake whichever address in your local subnet, but those are very likely remapped anyway in your gw to the one given by your ISP.

If you would have access to the switch port used by your ISP in the Internet exchange point (IX), you would have more liberties in choosing the IP.