Bubble mailers and shipping?

Okay, so I'm going to be mailing a GameBoy Color game to somebody, and I want to use a 4x6 bubble mailer. My question is, if I go to the post office do I have to pay for a bubble mailer, as well as the cost to ship it? Or is the bubble mailer going to be free and I just have to pay for shipping? 10 points for the best answer. Thanks!