The terminology is confusing. Under the PPWR, a “producer” is not necessarily someone who manufactures packaging. The regulation also covers businesses selling packaging or packaged products directly to end users in another Member State. So a maker shipping a board in an envelope can indeed be the packaging producer for EPR purposes.

Having looked into FAQ[1] it seems that it's mostly the case if you sell in branded packaging and are >10 people or >2M eur revenue.

[1]https://op.europa.eu/en/publication-detail/-/publication/ad1...