The deal on printing the label yourself is only valid in 2020. Don't know whether they will extend it.

"Selber online frankieren und als Privatkunde von einer Preisreduktion zwischen CHF 1.50 - 3.00 profitieren (gültig 2020)"

But the label always shows the official amount, not the reduced amount.

We send quite a few parcels and were having some difficulty with the thickness. My "friendly" (she actually was not generally that friendly) local post office manager gave me some calipers. I would go into the local post office and ask, maybe you'll get lucky!