Articles 28 and 29 are to do with import duty, and essentially say that goods coming into the EU from elsewhere in the world only need have duty paid once. Thereafter they can travel round other member states without attracting any other fees. As long a duty has been paid in a member state somewhere, that's it - no more duty.
Your sticker means that some bureaucratic minion with a roll of yellow stickers on his desk believes that although the package came from within the EU, the goods inside originated outside of the EU and that duty may not have already been paid by an EU member state - which in practical terms means that you may be liable for duty. However, Royal Mail rarely deliver anything with a fee attached without getting the money first, so the fact that you weren't asked to pay duty before Royal Mail delivered it means there is likely to be nothing to pay, probably due to a low content value.
I don't know the cost of mankinis, but you obviously managed to get a cheap one.