The OP does have a legit question regarding legal obligation under the FCBA that is totally separate from the responses that point to moral obligations.
Did you ever make payment in response to the first billing that posted the charge to your account and set a due date?
Are you asserting that the first bill was essentially negated by a subsequent bill that stated there is no amount due?
In that event, you may be able to require sending of a new, corrected bill, and prevent any negative actions, including credit reporting, until your dispute process has been concluded.
You have the right to file a formal dispute with the creditor under section 161 of the Fair Credit Billing Act within 60 days of the billing error.
That then requires their response and treatment under the procedure set forth in section 161 of the FCBA.
You can assert no legal obligation to pay until you have received a corrected and accurate bill.
You placed an order, rather expensive.
They submitted the charge as pending until they shipped.
They likely mistakenly submitted a new charge rather than updating the pending charge.
Your CC blocked the second submission and the pending charge timed out.
Do YOU believe you're not legally obligated to pay?
You received goods, you ordered, you gotta pay up.
If they sent you goods you did not order, you have some legal ground to keep without returning.
As you have alluded to, this site has credit experts.
We don't have legal ones, or if they are, they're here in an unofficial capacity and all that entails.
Even ignoring the ethical question, I think Jamie stated it correctly: this is unlikely to go away, and ultimately you are likely have to wind up paying for or returning the item... why go down the road of opening yourself up to legal challenge and what that entails both in the time domain as well as the financial one?
If you want real advice, go talk to a lawyer, this isn't a credit issue.