Yes there is - I used it myself back in April.

It's on the "main" level - where all the departure gates are, and IIRC, it's near the bit where you can look down into the duty free shops. Sells all sorts of envelopes and packing tape too, but looks more like an office supplies shop than a post office!

(oh, they were also happy to take USD's for payment, as opposed to Dihrams).