It would be interesting if someone could show that it was not Oswald's writing.
You can show whatever it is you're biased to show with handwriting "analysis".
But it would not change the evidence that Klein's received a paid order with Oswald's address, that it recorded C2766 as filling that order to send to Oswald's box no.
Since there is no evidence of such a package ever being shipped through the postal service or actually delivered to that post office in Dallas or actually picked up by Oswald or anybody else, then it's nothing but an assumption that he ever did.
and that Oswald held a rifle that cannot be distinguished from C2766 a few days after it should have arrived at the Dallas Post office if it had been sent as Klein's records showed.
The rifle in the photo "cannot be distinguished" from lots of potential rifles. That means absolutely nothing.
You seem to think that Klein's records have no evidentiary value.
It depends what it is you're trying to use them as evidence for. They don't tell you, for example, who shot the president.
I suspect that if Klein's records showed that C2766 was used to fill an order for a rifle to be sent to a PO Box belonging to a CIA agent, you would be insisting that Klein's records provided very good evidence (which, of course, would be true).
I suspect that making up strawman arguments is a poor method of argumentation.
They don't say that. They say that it was used to fill an order to be sent to Oswald's PO Box.
And you're also implicitly assuming that Lee Oswald was necessarily the only person with physical access to PO Box 2915. Even if it was delivered there (and there's no evidence of such), that doesn't mean Lee Oswald picked it up.