Question: my question is quite simple :
does google have an in app purchase service which , after the purchasing process is complete , will download the purchased content ?
if not , is there any other alternative?

the reason for this is to make the app as small as possible instead of having all of the purchased content within the app .
a possible solution for this would be to use the extension library and download only a partial chunk from there , but that's too complex and for most devices the entire extension will be downloaded while downloading the app itself through the market.

Answer: 
What I've frequently seen is to provide your extra content as another app download in the playstore, which then uses IPC to communicate/transfer that extra content to the main app.

I've seen video players, for example, request that you download a secondary app to get specific codecs. Once the user has downloaded the codec app, and it has transferred its codecs to the video player app, the user can remove the codec app.

In your case, you'd have an extra step of performing a managed in-app purchase before your app would agree to communicate with the extra-content app.

It's a clunky user experience, but other than paying for a dedicated web host to serve your extra content, it is the only option at this point.
Yes.

It is also fully documented on android's developer repository, complete with guides on how to set it up, etc. Obviously if you utilize google they will take a cut. But if you made your own, you would still have to give a portion to your credit card processor. Best of luck:

<http://developer.android.com/guide/market/billing/index.html>

Amazon's android app market also offers a similar service.

**EDIT**

"---after the purchasing process is complete will download the purchased content" :: NO <-- to that part. You have to handle that with your own webserver. Only the original app content is hosted on Google Play for you.