I have never heard of an antique trunk weighing less than 20lbs? Per OU..."Items must weigh less than 20lbs and be smaller than 108 inches in length + girth.".
This is just the forum and most of just are end users as well. Your post is rather confusing, but since you paid via OfferUp, your money is safely held in an escrow account and out of reach of the seller, until the transaction is finalized.
Since you have already made contact with a customer support, there is no further assistance available to you, as there is no number to call.
Here are the scenarios:
If you receive the item and satisfied with it, the seller gets paid...done.
If the receive the item and is not as described, you can file a buyer protection claim. If you win the claim, you will be refunded.
If you never receive the item, the transaction automatically gets canceled, and you receive a refund.
If the trunk is in transit, it will be disadvantageous for the seller to cancel the transaction, as you will receive the trunk and be refunded as well.
In theory, the seller can opt to use the USPS package intercept service, so that you do not receive the trunk, but your money will be safe and be refunded to you.
Just stay your course, as the OU rules are in your favor. As I have indicaed, your money is out of reach of the seller, so you are in control.
If the box is empty, there will be a weight difference from the USPS scale marking. File a buyer protection claim and you will win.