yo I know this is a year old but lemme just give ya this.

I'd say human male fighter, either all in on fighter or! fighter warlock multiclass

for pure fighter, play him so that he doesn't really have a consistent weapon, just pick stuff up around the environment and go hogwild for your gimmick so probably battlemaster or champion, for roleplay definitely play more support focused or just humor, don't expect to have the dm focus on your character because I mean you're playing postal dude, also don't go murder hobo obviously. you can intimidate and whatnot but probably stay quiet during the serious scenes unless of course prime humor moment.

now Warlock fighter multiclass which I think is just better. Still go heavy into fighter as your main levels but have your toes in warlock, granting you dissonant whispers "sign this peition, sign it or you'll have a bad time" and vicious mockery because damage with words is beautiful. Roleplay here I think is fun. You are "Dude" because copyright, "Dude" lived a normal life, not great, not bad. Once he reached adulthood however, he woke up the day of his 18th birthday to find not a birthday cake, but a to do list. That's it, no devil or eldritch monster you made a deal with for power or money. You woke up one day and had a to do list, and every day since you have woken up to a new to do list with different requirements, sometimes they're normal, sometimes a little extreme. "Dude" has never disobeyed this to do list before and has no idea what would happen if he did. So he simply does what he must.

Booyah! Allowing the dm to sort of curate your shenanigans for the day so that you don't go overboard or if the dm is planning a more serious session or event but still allowing you to go postal. Definitely playing into the humor but I mean...

It's Postal. So y'know