The problem has nothing to do with the postal service other than a surge of mail with a delivery timeline. The surge of mail isn’t a problem per se it is the delivery deadline for counting ballots. The postal service handles surges every Christmas and Mother's day among others. Some states have a delivery deadline that ballots must be delivered before a certain time. Some state’s were not allowing extra time for delivery so they were notified ballots might not arrive in time.
The problem comes with how to validate ballots are from real live voters. Absentee ballots are requested by the person. Mailing out ballots randomly whether the person is still alive or living at the listed address opens up the ability to commit Vote fraud On a massive scale as was recently found in New Jersey. To say mailed ballots are without increased risk of fraud is a lie. Period.