Casting back to Mr. Lunt's eighth-grade grammar class, a person would
LIE down, but he/she would LAY an object down.  (I LIE on the bed, but I
LAY a book on the bed.)

In this situation, "lie" is an intransitive verb (has no object), while "lay" is
transitive (must act upon an object).  ("Lay" CAN be transitive, but only if
you have the ability to produce eggs....but let's not get into that.)   As
your sentence is an imperative command (with an implied "you" as the
subject), in which a person (you) is acting upon an object (the crate), I
would say LAY FLAT TO UNPACK.

Mr. Lunt would be proud.  :)

Nickie

__________
Nicole M. Bouvier
Scheduling & Exhibitor Relations Director
Smithsonian Institution Traveling Exhibition Service (SITES)
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202.357.3168 x120  phone
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