Postal 2 is free on giveaway from GOG.com and is available to claim until December 18th, 2pm UTC. Check the Postal 2 PCGamingWiki article here, there are a ton of fixes for widescreen, field of view, multiplayer and co-op features.
Postal 1 is free permanently, nice! And of course, check out the latest in the series Postal 4: No Regerts, which was released in October 2019 as an 'early access' title.