Regular and Registered mails including Small Packets in Istanbul:

Post Office > 150. YIL AYPİM, Üsküdar (Anatolian side collection) or AVPİM, Zeytinburnu (European side collection) > UPİM, Bakırköy [the last stop]

Parcels in Istanbul:

Post Office > AYKİM, Üsküdar (Anatolian side collection) or AVKİM, Arnavutköy (European side collection) İstanbul KİM, Arnavutköy > İstanbul UKİM, Başakşehir [the last stop]

Key to abbreviations:

AY… (Anadolu Yakası = Anatolian side)

AV… (Avrupa Yakası = European side)

..PİM (Posta İşleme Merkezi = Mail Processing Center)

..KİM (Kargo İşleme Merkezi = Parcel Processing Center)

U… (Uluslararası = International)

If mailed outside of Istanbul, regional mail processing centers transfer mail pieces to Istanbul, the final hub [to the last stops], where the international mail leaves the country.

All the air mails and parcels leave the country via İstanbul Atatürk International Airport (main hub).

All the surface mails and parcels leave the country via ports by sea, stations by railroad, and by road from the last stops.

For the inbound international mail transfer flow, you may expect vice versa. However, the heavier small packets may handle like parcels.