Note Google still has the option of appealing to EU's top court, so the appeals process isn't even necessarily done I'm this case.

I hope they do and that the fine gets doubled as a result. Companies always appealing to the highest courts is only a function of the amount of cash they have, whatever happened to: "Judge says so, so that's it". It's just trying to do an end-run around the system by always clogging up the courts from low to high.

Honestly, just adding a market-rate interest charge on the original fine seems Good Enough to me. I suspect the interest on a $3B fine is larger than the court fees and lawyers anyways, and it's not like time value of money isn't something companies will neglect to take into account.

Wasn't the market-rate interest actually negative in some European countries in recent years?

There has to be some mechanism for some additional cost for the defendants to prevent appealing all court decisions...

If the interest was negative, then paying the money now is additional cost compared to paying it years ago.