I'm new to this year and I wonder if there is any kind of postal or logistics service which can send my packages to somewhere. Of course it would be perfect if they can help in packing them as well. Does anyone know about this?

You want to ship packages out? No problem. I believe there is a kiosk in the convention center, and a few of the connected hotels also have services.

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Alec Usticke, Unofficial Gen Con Indy Facebook Discussion Group

There is a UPS office in the ICC across from the VIG lounge. 
I've seen a couple of FedEx offices in adjacent hotels (JW for sure, can't remember the others specifically). 

didn't see it last year, but I do recall UPS having a booth set up just outside the dealer room.

The booth outside the dealer room is (I think?) for dealers and is part of the dealer support service.

The one in the ICC is either room 111 or a small, unlabeled room right beside it.

There's a FedEx store in Monument Circle, but there's probably something closer.

There's also a huge building on Ohio Street that's labelled "UNITED STATES POST OFFICE", which quizzically DOES NOT contain a post office.

There's a post office at South and Capitol, basically next door to Lucas Oil Stadium. 

I did just see a sign in the convention center that there would be a RICOH business center with, among other things, shipping services.

I recall someone saying there wouldn't be UPS in the convention hall this year (I might be wrong).

There is a Kinko's Store in the JW Marriott I think on the mezzanine level, which is connected by walkway to the northwest side of the convention center.

If you're shipping TO Indy you can probably just have it to delivered to your hotel and just give them a heads up to look out for it. I went on a business trip to England a few years back and we shipped all of our merchandise to the hotel, they even helped with shipping it back to Indiana for us.