Yes, you are within your rights to insist on the return of the $100 (not a credit; you can agree to a credit, but are not required to accept it) and the overdraft fee, which they are liable, or legally responsible for, because they had no right to do what they did. That said, there is no good way to get the money but by suing; and if you sue, even in small claims court, you'll have to give up at least a day of your time and pay a filing fee, which you may or may not get back, even if you represent yourself "pro se" (as your own attorney). Sometimes, you may have to accept the loss, because there is no practical route to compensation.