weird. my -15€ power usage I just used says otherwise

They are fixed (in total ca. 20 ct/kWh) so they are simply added to the negative price making it less negative.

Negative prices happen all the time, prices in ACER (Europes power market) are calculated on a 15 minute cycle (used to be 1hour) and your provider will be charging you negative prices if the electricity price is so low it overcomes the network fees. I've seen this exact thing happen on my own power bill.