I think that if Franz Kafka ever had to pick up a box from the Istanbul Customs office, he would have had enough fodder for another novel. I think he absolutely could have penned a terrifying and disorienting novel called The Post, in the same vein as The Castle or The Trial.
Because having to pick up a package at the Istanbul Posta Müdürlüğü is no easy feat. I've had to do it a few times since moving to Turkey, most recently a couple of weeks ago to pick up a Christmas box from my mother-in-law that had been languishing at the Customs office for three weeks with nary a note from the post office saying that it had arrived.
I don't know why the Customs office refuses to send notes saying that a package has arrived and you must come and pay a tax on it. The only way that you can track a package is by getting a Customs number from the person who mailed the package to you and then plug it into this website (in Turkish) to see where it is in the Turkish system. After that, though, you then have to actually call the post office -- 0212 565 1400 -- and ask what reference number they gave your package so that you can go pick it up.
Don't be surprised if, when you call, the phone is either busy for hours on end, or it rings for ages without anyone picking it up, or you are immediately routed to a phone maze in which pressing zero multiple times does not connect you to an operator but instead disconnects the line. Keep trying. Eventually, even if it takes several dozen attempts, someone will answer the phone with a very angry "Alo?!".
The reason I had to physically pick up my mother-in-law's box from the post office was because the declared Customs value of the items in the box exceeded 100 euros, the limit in Turkey for Customs-free boxes.
And because the box was valued at $204, which was well over the $130 limit, I had to pay a Customs tax on it.
This is why you are supposed to either 1) Lie about the value of the box on the Customs Declaration Form; or 2) Separate the items in the box into several, smaller boxes so that the value of each box is listed as under 100 euros. (I highly recommend option 2, though I know many, many people who employ option 1.)
If you've ever had to go to the post office and pick up a box at Customs, you know there's something very Kafkaesque about it.
Upon arriving, push your way past the gaggle of people languishing in front of the entrance and go to Window 1. Show the employee the slip of paper on which you wrote the parcel number that you got earlier on the telephone. No words are necessary here, as the man will sort through his shoebox of tissue-thin paper to find the original Customs Declaration Form that they removed from your box.
Go to Window 8. You thought I was going to say 2, didn't you? Ha! I told you this was Kafkaesque. At Window 8, hand the Customs form to the gentleman behind the window. This employee will find the box for you and pass it along to his colleagues. Stand around idly while you wait for your name to be called. When it is, you will watch as another employee opens the box and inspects the contents. Then, a third employee will calculate the Customs tax you have to pay, which is based on the value that is written on the Customs form. I don't know the formula for this, but for my box, which was valued at $204, I had to pay 63 TL.
Now go back to Window 1. Hand the man there all the slips of paper you now have. He will stamp a few things and give you a few slips of paper back.
Take all of these to Window 2. Here is where you pay the Customs tax. Only cash is accepted and exact change is required. Annoying? Quite. Oh, and you can't find out in advance via telephone how much tax you have to pay. The only way to find out is to go to the Customs office yourself.
Now go to Window 9. Show the gentleman here all the slips of paper you have accumulated. Show him your passport. Pay him 2.05 TL. 1.50 TL is a clearance tax and 0.55 TL is a stamp tax, though if you ask him why you have to pay this money after you've already paid Customs, he simply stares at you. Best not to ask.
Go back to Window 8 and give the employee there all your little slips of paper. Within a few minutes, he will emerge with your package and voila! You are done!
How to get to the Istanbul Posta Müdürlüğü (Davutpaşa Caddesi No. 99, Cevizlibağ, next to the Ülker Fabrikası): You can take a bus (most likely the 92T) from Taksim but since I'm a full-time working girl I don't have loads of time to waste during workhours on public transport so I take a taxi from Taksim Square (22 TL one-way, about 15-20 minutes) and a taxi on my return. Don't bother having the taxi driver wait for you while you get your box; there will be plenty of other taxis you can hop into on your way back.