This is one of those questions where you'll probably get a million different answers
I have heard you can present them at the purser's desk and receive the credit. I would not chance it, I would do as the instructions say and mail it in. But that's just me, I'm paranoid something would go wrong.
You will receive a check on your last night for any amount you didn't use.
I've used them but always mailed them in as stated. Good luck!