Komkrit "Soranat" Wongsasing took Bt299,674 from 24 money orders between November 16, 2000 and March 2, 2001 when processing money orders at Dusit Post Office.
Komkrit, 32, has returned the money to Cat Telecom, which organises money orders via post offices.
"I was too young to realise I was doing wrong. I was worried about taking care of my elderly parents," he said.
Because Komkrit pleaded guilty, the court commuted his sentence from 100 years.
He was convicted of abuse of authority and corruption.
Komkrit was arrested on November 18 last year.