They don’t need to hire more mail carriers.
They need to switch to every-other-day delivery, except perhaps for high-volume commercial customers.
Half of all routes would get Monday-Wednesday-Friday delivery, the other half would get Tuesday-Thursday-Saturday delivery.
The amount of mail is the problem. Not enough postal workers are hired to deliver it all, so the carriers work 12-hour days, six days a week. On Sundays, they still go into work half a day.