If POSTAL generated the same gross revenue on the Epic Games Store, Running With Scissors would keep approximately $15,773,762 - that's $2,932,488 more than on Steam.
At this revenue level, Epic's concrete platform fee would be $2,014,604. 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 $12,451,856.
Note: This is a hypothetical comparison. Actual sales volumes differ significantly between platforms, and Steam's larger user base typically drives substantially higher sales.