Here are some words that will come in handy at the post office (postane):
To send: gOndermek
To mail: postalamak
Registered: taahhutlu (tah-hoot-loo)
Stamp: pul, posta pulu
Postcard: posta karti
Often, if you merely be polite and hand the person what you need mailed, they will take care of it for you and give you a total. Packages will be sent airmail as a default, and so unless you wish to have something registered with insurance, you can get through it with very little Turkish if you need to.