If POSTAL 2 generated the same gross revenue on the Epic Games Store, Running With Scissors would keep approximately $25,828,425 - that's $4,417,836 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $3,385,694. Epic charges 0% on the first $1 million in annual per-app revenue and then 12% on the remaining amount. On GOG (30% cut), the developer would earn approximately $20,449,884.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.