The word excise also means delete, so I wouldn't confuse her talking about excise charged on the item (as in a tax). She just means take the postage off.

Just check again that you did remove the postage from the invoice you sent. Sometimes people click "recalculate" to change the invoice and it changes it back to the original showing the postage. The better button to press is "preview" so you can see how the invoice looks before you send it.

So I'd send the invoice and if you haven't heard in a couple of days (not more than 2) open a dispute.

That usually either gets slow payers moving, or gets you on the way to resolving the situation and being able to re-list.