That depends on the tax.
Sales tax is not "paid" by a merchant, it's collected from the buyer and forwarded on to a taxing authority.
If the seller is keeping books, the $15,000 shipping charge would probably result in some sort of profit, which _can be_ taxable, depending on the structure of the business and the conditions on how the money is taken out of the business, such as owner's dividends, salaries, etc...
Barry (being stupidly hypothetical, since we've moved to the stupidly hypothetical world...)