Percentage of mass is probably the wrong metric to look at, because it assumes that the USPS could simply eliminate the X% of mass used by junk mail and save roughly X% on fuel/delivery costs.

But of course the issue is that the junk mail is subsidizing the actual mail. There's likely no way the USPS could be financially solvent, at least with the current level of service, if junk mail were eliminated. Personally I'd be fine with that. One or two mail deliveries per week would be more than enough!