How are you supposed to send certified mail?

i made a payment of ten dollars for a license i need and they gave me back two papers to send to sacramento, one is a white paper that says us postal service certified mail receipt and the other is just a green paper...am i supposed to go to the mail office and pay them to send my mail or is it already pay for???? i did pay ten dollars already so that's why im confused...any info on this would help, thanks