EDIT: after reading the rest of the comments OP added that these prices include their 20 cents/kwh fee.
One thing to note, these spot prices are the price before any fees, your network fees and all that are per kWh and usually more than the negative sum the electricity price goes to.
(probably in this case though -38 cents is a lot)
Negative prices usually only truly exist for enterprise customers that pay 1/5 of the consumer network fees.
EDIT: after reading the rest of the comments OP added that these prices include their 20 cents/kwh fee.
One thing to note, these spot prices are the price before any fees, your network fees and all that are per kWh and usually more than the negative sum the electricity price goes to. (probably in this case though -38 cents is a lot)
Negative prices usually only truly exist for enterprise customers that pay 1/5 of the consumer network fees.