Good point -- I don't know how to estimate the marginal cost of delivering marketing mail but it's certainly not zero. Looking at the same USPS FY2020 financials [1] it looks like marketing mail constitutes a little more than half of the pieces they deliver. That about matches my experience. My guess is that several times a month I have a mailbox that contains solely marketing mail. I'm not sure how much money it would save the USPS if the mail carrier passes by my house without stopping on those days, but I suspect the bigger impact would be dealing with half the physical volume of mail for our household.
I still hope we can find a more sustainable path for the USPS than forcing them to stay alive by delivering something no one wants to receive.
[1] https://about.usps.com/what/financials/integrated-financial-...