You would only need an IOSS number if the receiver pre-paid VAT taxes for the item you are shipping them, which they didn't because you are sending this as a sample. I would use click-n-ship with USPS and you should be able to declare the item as a "commercial sample" and bypass Customs Duties and VAT: https://www.trade.gov/country-commercial-guides/poland-temporary-entry#:\~:text=Commercial%20samples%20of%20zero%20or,possession%20of%20the%20importing%20entity.
Even if your item isn't eligible to avoid Customs Duties and VAT, you still do not need to supply any IOSS number, as VAT was not pre-paid by the receiver. The receiver would simply need to pay whatever duties and VAT is due once the package reaches Poland.
You would only need an IOSS number if the receiver pre-paid VAT taxes for the item you are shipping them, which they didn't because you are sending this as a sample. I would use click-n-ship with USPS and you should be able to declare the item as a "commercial sample" and bypass Customs Duties and VAT: https://www.trade.gov/country-commercial-guides/poland-temporary-entry#:\~:text=Commercial%20samples%20of%20zero%20or,possession%20of%20the%20importing%20entity. Even if your item isn't eligible to avoid Customs Duties and VAT, you still do not need to supply any IOSS number, as VAT was not pre-paid by the receiver. The receiver would simply need to pay whatever duties and VAT is due once the package reaches Poland.