It may be marginal if you receive other mail on a daily basis. Personally, in an average week, three of the six mail delivery days, I receive nothing but junk. Even if the marginal cost difference is one penny per household (think time and fuel as well), there are 130M such households in the USA. Marginal for one home, maybe, but for millions of homes it adds up. Margins count when you're trying to effectively run a business, not even necessarily to earn any sort of profit. In the case of the USPS, just to break even.
I think the basic point of this article is either change your business model, or get out of the way and let someone else do it better. Otherwise you can't exactly complain about losing money.
Except that since most of the mail anyone receives is junk mail, most of the resources go towards handling that junk mail. Ridding the US of "spam" snail mail would be advantageous because it would:
1. Generate much less waste
2. Utilize far fewer resources at the USPS
3. Allow the USPS to cut some fat and run more efficiently
So, while the revenues would decrease, the costs would decrease as well.