On the USPS’s site, at one point it asked me
Enter the ZIP Code from where this item may be mailed:
What is it asking for?
The first answer. Since it is generally/technically ungrammatical to end a sentence with a preposition, the construction of their sentence isn’t wrong, nor is it confusing. What about it is confusing for you?
Probably should be “from which,” but it’s perfectly understandable as written.
The zip code within which you will run screaming toward the nearest postal worker and throw your item in their general direction.
You can mail an item from a place, and you can mail an item to a place. They’re inquiring about the former.
If they wanted to know the destination ZIP code, you would expect them to say:
“Enter the ZIP code to where this item may be mailed:”
It should be “from which” and not “from where”, but it still makes sense.
Hmm. I like it better with “from where.” What I don’t like is “may.” I think it would read better like “Enter the ZIP Code from where this item will be mailed.”
I could go either way on the “which/where” debate; I agree with **Leaffan **that the thing that I don’t like about the sentence is the word “may”.
The only reason I can think of for this (in my opinion) rather odd phrasing is that if they used the word ‘will’, this might imply some sort of commitment on the part of the user, and they don’t want to do that.
Well up with that language I will not put!
I don’t think it’s necessarily right or wrong to end a sentence in a preposition but I can understand how USPS wouldn’t want to. (Wouldn’t to want? To wouldn’t want?)
I think a less confusing choice would be “From what zip code will this item be mailed?” That’s perfectly clear. Even changing “may” to “Will” as some posters have suggested would do the trick.
The way it’s written right now, it sounds so unnecessarily proper. Like the website wants to know where it’s going but is asking my permission to mail the item. It’s just weird.
This might be it. It’s on a page about a service (Global Express Guaranteed or GXG) that is only available at some post offices. So you would want to say that it “will be” from a Zip code if the only post office in that Zip code doesn’t offer GXG.
The “may” is the confusing part.
Agent Bork: Chief, you know that guy whose camper they were whacking off in?
Agent Fleming: Bork, you’re a Federal Agent. You represent the United States government. Never end a sentence with a preposition.
Agent Bork: Oh, uh… You know that guy in whose camper they… I mean, that guy off in whose camper they were whacking?
Option 5:
Enter the ZIP Code whence this item may be mailed:
The only thing I see wrong is that you could enter the ZIP code of any post office that would mail the package since it says ‘may be mailed’, not ‘would be mailed’.
To is not a preposition in that sentence. It’s an elided infinitive. (I don’t know if the word “to” has a specific name). The full sentence is “I don’t think it’s necessarily right or wrong to end a sentence in a preposition, but I can understand how USPS wouldn’t want to do so.”
Same here.
And there’s nothing wrong with ending a sentence with a preposition. Even early 20th century language mavens like Fowler and Strunk and White have no problem with it. Fowler even derides the rule as a “superstition.”
Would not so desire. 
“To” is not a preposition there: it’s a particle that is often attached to the infinitive form of a verb (“end” in this case, which is left out because it’s understood).
“Enter the origination zip code.”
“Enter the destination zip code.”
I voted option 4. It’s a horrible construction.
The ‘from’ could mean ‘belonging to’, so:
Enter the ZIP Code belonging to [the location]where this item may be mailed [to]:
Conversation with my wife last night:
“I wish you hadn’t put that thread up on the USPS today.”
“Why?”
“Because it seems so harsh that you got completely schooled on your birthday.”
Well the Dope has spoken and in overwhelming numbers that choice #1 is correct. The thing is, I believe that the phrasing -while grammatically correct- is unnecessarily complex and confusing. I think the comments here have shown general agreement on that point.
Instead of “may”, which sounds like asking permission, how about “might”, which conveys possibility?
“Enter the ZIP code from where this package might be mailed:”
They can’t go with “will” because then some people might think “Oh, if I enter the zip code, it’ll confirm the purchase” or whatever. I like my version best.