score:7
For the question in the title:
Schengen has nothing to do with customs ... but Poland and the Czech Republic are also both in the EU, which is among other thing a customs union.
There are a few categories of goods that cannot be transported completely freely between the EU countries, due to internal consumption taxes in the various states -- but as long as you're not traveling with largish quantities of alcoholic beverages or tobacco products, you shouldn't need to care about customs when crossing the EU internal borders.
For the question in the body:
As regards movement of persons, the Schengen Area is (still, supposedly, officially) one common travel area with no internal border controls. The entry stamp a foreign traveler gets when entering the Schengen Area means that the Schengen Area has been entered, not just the state whose particular external border the area was entered through.
The exit immigration check in Prague will look for your latest Schengen entry stamp to determine if you have stayed for too long in the Schengen Area. They will not care about where you entered the Schengen Area as long as they can ascertain when and verify that you abided with the 90/180 rule.
Upvote:0
If you have nothing to declare, you should be just fine. No need to see anyone.