I think your customer has experience.
Those sides either get rolled or folded a lot.  
Even a  perfectly made banner (vinyl/vinyl) will look like crap after being rolled and folded 
a few dozen times.
For the purposes of flexibility (and non-cracking) I'd be thinking a water-based 
sign paint on 
those sides once the surface was properly cleaned and prepped.  
I am far from an expert at that so I'll be watching the replies in this thread.