And to be fair, he did get a literary hand out from John Martin. Sure, it was only $100 a month, but he wouldn't have written his first novel without it.
Post Office wasn't the first novel Bukowski wrote (though it may be the first one he finished).
As for the money Martin offered to pay Bukowski, if I offered you $600 a month right now to quit your job, would that do it for you? Could you make it on that? Because that's what the famous $100 a month would be in today's dollars. Not enough to pay rent on the shittiest ghetto apartment anywhere within a hundred miles of Los Angeles, let alone rent, food, liquor, stamps, envelopes, paper, typewriter ribbons, taxes, insurance, child support...
The money Martin offered Bukowski didn't hurt him, that's for sure, but he also didn't need it to survive. It's unlikely that he could have survived on it even if he wanted to try (it was less than a sixth of what he made as a postal clerk). He had substantial savings, and it was more likely those savings that allowed him to quit the post office. The offer from Martin was more a show of faith than financial support.
But I don't think there's any question that the show of faith meant quite a lot to Bukowski. Much more than some small sum of money that has now been mythologized far beyond it's real and actual importance.