This is an industry-wide issue that has been extensively researched by our Production team. We've found that the USPS's high speed sorting machines are what typically cause these scuff marks. If postcards or self-mailers get exposed to the belts for a millisecond longer than they're supposed to, they're at the risk of getting abrasions.
This is not an isolated incident for our printer or for our mailers. Here are a few articles that further explain the issue.