The private American mailing companies will literally charge as much as they possibly can to send a package to a foreign country - costs for airplane fuel, truck fuel, salaries of the people needed to transfer the package from point A to B to C, and so on. But they will usually send their own couriers to make sure the package gets to the destination. Once you drop the package off, it is in the company's hands exclusively until it is delivered by hand. For this, you will pay money.
The U.S. post office, however, is subsidized. International costs are even less thanks to a "cost sharing" program that all nations' postal systems have with each other. But, you have to take the risk that the post offices, in the U.S., the countries of transit, and in the destination country will actually keep track of your package and make sure it gets delivered properly. Even registered packages aren't guaranteed 100%. Probably the only way through USPS is to send an express package, since that is given the highest and only real priority.
Again, I have no experience with Poland, but I have sent letters and packages, and have received letters and packages, to/from several different former USSR, some of which are far poorer and more disorganized than Poland, and I have never had any trouble. The largest problem I ever had was a letter that was delayed by 2 months - that is all.
Just make sure the address is exact, and that all the post offices from drop-off to pick-up and in between can understand exactly where it is going. Have your paperwork (customs forms, return receipts, etc.) all in order, in English (if sending from the U.S.), and preferably in the language of the country's destination, too. If nothing else, French is still the international postal language.
If the postal system in the destination country isn't sure about something, they WILL open and examine the package for customs and security reasons. This is guaranteed to delay what you are sending by quite some time... assuming it will ever make it out of the customs office again!