You can reissue it, but you asked for the invoice and the merchant didn't send it to you. You can ask the merchant to issue an invoice and mail it for free (this still depends on the attitude of the merchant). But if you forget to ask the merchant to invoice you, you still have to pay the postage yourself. After all, it is your own fault (but if the merchant is good, it can be sent to you for free, depending on your relationship with the merchant).