I get that it could cause a lot of lag, and potentially even crashes, to bundle that much data into a single block.

My suggestion for a revision is to allow specifically just packages to be placed into another shipment package.

For some people, shipping 6 items isn't enough. Being able to add up to 6 packages to mail a total of 36 items would be quite nice, especially for roleplay purposes where everyone mails something to a central post office to be forwarded elsewhere, since you could bundle up to 6 packages to a single person into one mail system.

To prevent people from stacking this infinitely, you could add a new package type that is created only when other packages get nested inside. The item could be called "Stacked Packages" and could visually appear as just two boxes tied to one another vertically, but could actually contain up to 6 other packages, or 5 packages and one other item, 4 packages and 2 other items, etc. and would essentially only get created if the mail you're sending contains another regular package.

That'd still be less data than a large chest, and if it proves to continue to overload data when being sent, you could always just store the contained data somewhere else, either in an unused custom sub-dimension with callback references that summon the nested packages upon opening, or just in the mod data - whichever would cause less lag. Other mods often can and do utilize the same techniques, with one example I can think of right off hand being Pepperfly's Sophisticated Backpacks and the Inception Upgrade.