In the "Trade-in bonus" tab, the following is noted:
"Trade-in on the complete iPhone 12 family with 10% trade-in bonus.

Benefit from a trade-in bonus on your old smartphone when you buy a new iPhone 12. If you trade-in your old mobile phone at verkaufen.ch, you will receive an additional 10% on the residual value. This promotion is valid until 16.4.2021."

So yes, send in your old device.