Most people do go to the post office to send things, but if they're small enough, you can just use stamps and put it in your mailbox. If you go to the post office, it shouldn't be more than $3.50.
I would take it to the post office. Often when using stamps, it's not enough and your receiver ends up paying the difference. I know I had to a couple times. I mail tops often and it usually averages between $1.50 - $2.50. Hope this helps. Good luck