USA, Canada / Shipping personal possessions from US to Poland [11]

Yea I definitely agree, you should use one of the Polish companies, it will be a lot cheaper for you.
I know lots of people that used Doma Export in Linden, NJ. They shipped their mienie by cubic meter so
it ended up not being expensive.
You can get their contact info on their website domaexport.com

USA, Canada / Shipping books / package to Poland from US [15]

I have shipped packages to an individual like you are writing and never had a problem. As long as value is less than $35 they should just deliver it to the door like any package. It wasn't with USPS though but with a polish company - Doma Export

domaexport.com
hope this helps :)