How many stamps do I need to send a letter from the US to Poland (it's in a flat light but slightly big envelope (about half a piece of paper))?

To send a letter from the US to Poland in a flat, slightly oversized envelope, you will typically need one Global Forever stamp, which covers international mailing up to 1 ounce. If your envelope exceeds this weight, you may need to add additional postage based on the weight. It's best to check with your local post office or the USPS website for the most accurate and up-to-date postage rates.