Sending small parcel from Poland to other EU country question

Hi Everyone!

Is it possible to post a envelope / post bag like this in an ordinary Polish letterbox with just stamps according to weight ?
Is there any any rules that you need sender address and sign when received in another EU country?

transpack.co.uk/media/catalog/product/cache/1/small_image/295x295/9df78eab33525d08d6e5fb8d27136e95/_/q/_q0v5953copy.jpg

Hope you understand my question ! :)
Read alot about involving Post office in Poland but is it needed ?

Not sure about it, but why go through so much hassle to leave the post office out of it. For packages you need to fill in a form, that is put on the package, and a copy goes home with you. I've sent a bunch of packages home in the last 2 years, and it really a 3 minute job at the post office. And to my standards, pretty cheap.

If you want to be sure it arrives, just go and post it at the postoffice.
A sender address is required, but ofcourse nobody is checking what you fill in there, if there is any reason to hide your send-from address.

You should go to the post office and send the parcel. After checking the weight of the parcel, a corresponding fee is set.
The service is helpful Parcel Forwarding Poland.