If you feel that you can show that the optometrist ignored you clear instructions, you may be able to sue them for negligently losing your glasses. It's an uphill battle though, since first you'd need to prove the instructions  were provided (whoever is suing has the burden of proof, to establish his or her case), and you'd *possibly* also have to prove that mailing the glasses back anyway was unreasonable or negligent (since if it was perfectly reasonable to mail them anyay, there'd be no liability). If you are willing to take  the time and small  cost (usually around $30 or so) to go to small claims court, can accept that winning is not guaranteed, and aren't worried about relations with this optomitrist in the future, there is probably no reason to not try small claims court--again, just understand it's not  at all a given that you'd win.