"Ein Extra für mich" doesn't sound German. Possible alternatives:"Ein Bonus für mich" "Eine Zusatzleistung für mich" "Ein Goodie für mich" "Für mich (eins) dazu" and many more depending on context.
It's a bit unnerving that for so many Duo sentences I read you say they don't sound german. Also, I love that 'Goodie' is a german word. Oh goodie!