I wouldn't go with the pre-cut mats. I'd want to 
be completely safe with my investment. With the 
mostly acid-free mats, the backing board and the 
core are acid-free, but not the surface paper. 
Over time the acid in the paper CAN 'jump' to the 
cel, or it can move down into the core and 
backing. That's why the bevel of those mats turns 
yellow over time. It's the acid. 

Even if the guy says they are 99%... I wouldn't 
risk it myself. But I guess that's also a luxury 
on my part, that I can afford the acid-free mats 
when I want to frame something.