The USPS or United States Postal Service is privately funded and receives no tax dollars to operate.
The USPS or United States Postal Services is funded through the sales of postage stamps and other postage that people pay to mail letters and ship packages.
All that mail and all those stamps being sold and used on a daily basis adds up to a lot of money along with the postage bought to ship packages through the USPS.
The Post Office operates on a yearly revenue of around $70 billion dollars which seems like a lot of money but once they pay for postal employees and the buildings and the vehicles and the gas or diesel in the vehicles to ship the mail.
The money isn't all that much but still the USPS is pretty profitble.