rabbut
I don't bite, all that often...
Well, minuscule amounts of copper can play havoc in a marine tank with inverts, where as most FW inverts will tolerate trace levels of copper reasonably well 
Most LFS's out my way charge 50p a gallon for fresh RO or £1 a gallon for pre-salted. You are better getting fresh RO and salting your own when you get to adding corals, but the salt RO option is good for water changes and the initial fill while you have just fish and mobile inverts. You need the fresh RO for topping up also
I'm not aware of addatives for copper that actually work for it's removal
There may be some, but getting them may be tricky. Of course, copper isn't the only toxin you use RO to remove, other things may be lurking in the water...
All the best
Rabbut

Most LFS's out my way charge 50p a gallon for fresh RO or £1 a gallon for pre-salted. You are better getting fresh RO and salting your own when you get to adding corals, but the salt RO option is good for water changes and the initial fill while you have just fish and mobile inverts. You need the fresh RO for topping up also

I'm not aware of addatives for copper that actually work for it's removal

All the best
Rabbut