How to understand おきます in this sentence?

Question

The following is an excerpt from a conversation between me and my language partner:

私の Skype ID を書いておきますので、もし良かったら登録しておいてください。italki と Skype のどちらでメッセージを送ってもいいです。今すぐ使わなくても、将来役に立つときが来るかもしれません。

So, about て-form おく, I know three usages:

1. Accomplishing a preparatory task for the future, like in:
   旅行のまえに、切符を買っておきます。
2. In preparation of future use, one accomplishes a preparatory action:
   はさみを使ったら、元のところに戻しておいてください。
3. You leave the state of something as it is:
   明日会議がありますから、いすはこのままにしておいてください。

I'm not really sure if one of these applies for the おきます construction in context of the sentence in question. However, here is my attempt at translation:

"Because I have to write my Skype ID, if you don't mind, please make a registration (Shall I register on Skype here or shall I "register" her ID???). We can send messages both on Skype and italki. Even not using it right now, the future point in time where it is of use will probably come."

What makes me so unsure about my understanding of the おきます constructions is that I can't really say how her "preparation" of writing me her Skype ID is in a relationship with my "preparation" of registering on Skype/registering her Skype ID? I can see the purpose of registering someones Skype ID with whom I want to talk via Skype. However, I have problems with the "preparation" semantics here...^^

Answer 1

I think it's much more reading comprehension. She says "I will write down my skype ID because I think it might be in need in the future even if you don't use it right now". This is kind of her preparation task since she thinks it might be useful for you in the future. And she says if you think it might be useful in the future, so feel free to register it.

I think you don't have to do grammatical analysis so much though, probably your No.1 & 2 applies in this case.

Answer 2

It seems that you talk with your teacher on italki or email, and he/she have sent you that message. Anyway, I will tell you what it means breifly.

私の Skype ID を書いておきますので、もし良かったら登録しておいてください。
I will write my Skype ID here(itaki/email), so please add me to your friend on Skype if it's OK with you.

Of course you shouldn't register with his/her ID. He/She mentions "if you don't have Skype ID, why don't you create a new account to talk on it?" So, you don't need to register on Skype.

As of the ~ておく, one of its meanings is preparation in advance for something. An example sentence is following:

メモを残しておく / I left little notes to myself.

Other parts of your translation is fine.