Seems really unfair that because the buyer threw away the packaging you cant claim on the insurance. Can you ask her if she can find the packaging? Maybe it's still in her bin?
& maybe explain to the insurers what happened & ask them what normally follows in this circumstance? Seems really unfair you cant claim because of her lost packaging.
If the insurere wont help, i guess you could try broaching the idea of asking her to let you repair the mirrors & you each pay your own postage costs....explain if she Can still find the packaging, the shipping would all be covered