You're gonna have use boxes, envelopes are a BIG NO!
We're talking about lithium batteries people, and a lot of them, so unless you want significant damages to your cells....I highly suggest locating boxes and crapload of newspaper!
Crumpled newspaper is a great packing material and if you collect your old newspapers, it's cost effective.
You'll also need to order bubble-wrap for wrapping the cells and I can help you locate the same stuff I used....if you need help?
Mindfield, if you wanna take on this responsibility....send me a PM, then we're gonna have to get the okay from Joker before I start giving you advise.