A man entered the store on Balmoral Place between 07:30 and 07:45, threatened a 33-year-old member of staff with a weapon and attacked him.
The suspect fled with a four-figure sum of cash, cigarettes and the wallet and mobile of a 61-year-old customer who came into the shop and intervened.
Police described it as a "shocking" and "vicious" attack.
The suspect was described as white, in his early 20s, wearing black trousers, a black top, light pink trainers and a black scarf covering his face.
He was also carrying a black backpack which may have had pink markings on it.
Det Con Andrew Hope said: "This was a shocking attack. It was vicious and the member of staff sustained a serious head injury during the attack.
"He was taken by ambulance to the Borders General Hospital where he was treated and has since been released.
"The suspect had no thought for the staff member or the customer who thankfully was not injured in the incident.
"The Post Office is a busy place, as is Gala Park where the suspect made off towards, so I would appeal to anyone who saw the attack and who hasn't already spoken to police, or anyone who can assist officers with their inquiries to contact us."

The Nottingham-born actress told The Graham Norton Show the incident happened in 2015 after a ceremony in the city to unveil the tram.
She said she and her mother had been told they could have a free ride but were later challenged by a conductor.
Nottingham Express Transit said it was an "unfortunate misunderstanding".
McClure is among several well-known locals to have trams named after them on the Nottingham city network.
Others include ice skaters Jayne Torvill and Christopher Dean, cricketer Stuart Broad and author DH Lawrence.
Speaking about the incident, the actress said attempts to prove who she was fell on deaf ears at the time.
McClure said: "We unveiled the name and then I got on the tram with my mum because we were doing Christmas shopping and they'd said 'we'll give you a free ride'.
"We were pulling into our stop and I can see the ticket man coming towards us and he's like 'can I get your ticket?'"
She said the conductor was unswayed when she presented the plaque she had received at the ceremony and ordered the pair off.
"I was like 'oh my god - you've thrown me off the tram for fare evasion. I was mortified," she explained.
Despite the misunderstanding, McClure said she wanted her name to stay on the tram for good.
Nottingham Express Transit, which runs the network, said: "This was an unfortunate misunderstanding as the travel officer was unaware that Vicky had been offered free travel on the day the tram was named in her honour.
"As you would expect from her, Vicky took the matter in good part and the situation was soon resolved.
"We apologised to her as soon as we were aware of the confusion and we certainly didn't want to spoil such a special occasion."
A shop worker has been seriously injured during a raid on a post office in Galashiels in the Scottish Borders.
[NEXT_CONCEPT]
Line Of Duty star Vicky McClure has revealed she was ordered off a tram named in her honour after being accused of fare evasion.