Greeting card companies frequently hire freelance writers for all types of cards, including birthday and well wishes. Card companies also hire freelance artists and photographers.
A freelance writer must apply to each company for which he wishes to write. Several companies accept writing samples on various topics and pay the writer a flat fee for his work if it is accepted.
As of 2015, Blue Mountain Arts seeks writing samples about love and friendship. Comstock pays $50 for each accepted adult humor idea. Freedom Greetings is open to submissions and It Takes Two considers verse if it is unique. JQ Greetings accepts verse submissions via email. Moonlighting Cards runs a contest for best submission, but payment only goes to the winner.
RealWaysToEarnMoneyOnline.com has information about writing for several other greeting card companies.