This is an industry-wide issue that our Production team has done extensive research on.
We've found that the USPS's high speed sorting machines are what typically cause these scuff marks. If postcards get exposed to the belts for a millisecond longer than they're supposed to, they're at the risk of getting abrasions.
This is an unfortunate issue that has continued to become more widespread as the USPS continues to experience higher volumes of mail, less staffing and funding, and longer periods between maintenance on or replacement of their sorting machines.
This is not an isolated incident for our printer or for our mailers. Here are a few articles that further explain the issue.