This is all wonderful advice, but I'm a real neophyte to all this and therefore dismally confounded by all the "Terminology" that manufacturers use.

Can anybody tell me the relative qualities and demerits of these products that I've come across online? I quote from the item descriptions, and they are:

--- Heavyweight, crystal clear, 4 mil Mylar D envelopes;

--- Self-sealing L-sleeves for Sheet Music, constructed of archival
    3mil archival polyester with no adhesives;

and

--- Perma/Dur® Sheet Music Folders, manufactured from 225gsm      
    acid-free folder stock that is buffered with 3% calcium carbonate.

Many thanks in advance!