The term "ACID FREE" is not a reliable indicator of long-term stability. This is why: the term is not a regulated one. Anyone can say that anything is "acid free" and it is not checked by any regulating body.
All the term "acid free" means in a reliable sense is that the product, when it left the manufacturer, was not of an acidic pH (i.e. below 7). It does not mean that the product will not develop acidity as time goes by. It is entirely possible that something called "acid free" will become acidic as it ages, due to what it is made from.
It's particularly frustrating when it is applied to plastics, where acidity is often not the concern. The old standard PVC (polyvinyl chloride) releases chlorides and plasticisers which discolour and damage things like photos in their vicinity. Acetates do release acid as they age, but again it's the plasticisers that are released as it ages that are the problem.
"Photo safe" is not a regulated term either, but it can be a better indicator of archival quality.
However, if the item has passed the Photographic Activity Test (PAT) then it has been tested by an accredited facility and is certified safe to use as packaging or housing for photographic (and other) materials.
I hear the acid free thing all the time in my job as a paper and photograph conservator, and it irks me that it can be used by disreputable firms to mislead customers into thinking they are buying an appropriate product.