Are you for real? you want to buy something from a Polish company and you can't because they want your NIP??? I think the average nip is 9 digits long. Just enter some numbers in.
NIP is a PL tax identification number. They shouldn't be asking for your NIP if you want to buy something as a private person. Now, I believe they could ask for it if you were buying on behalf of a company or asked for an invoice for a company.
I pay ZUS and I only have a pesel number. I do not have a NIP number as I do not pay taxes in Poland, the Regon number is for a company, not a private individual.