I know media mail is the cheapest and slowest rate for books but .  .  .  . does it depend at all on the size of the package? I've heard they throw media mail on the truck when they "have room" for it.

So if I have a really large book order from a company that's shipping media mail should I expect it to take the full 4 weeks? It might make a difference in how I ship them.

Thanks!

I doubt it'd make any difference at all.   I sent a package once with a ton of books in it and it got there about the same as any media mail.

From my experience - there's no logic to Media Mail.   And i doubt the size of the package/box would matter.  

I find that PO clerks automatically chuck the books into the "penalty box" - I can't recall the last time a book took more than a week, often faster, when using APC postage.

I don't think it's the size of the package that counts, since I don't think PO trucks are loaded by the individual package. Probably they throw crates onto the truck when the have room for it. It'll just be as unpredictable as "regular" sized Media Mail. =)