Worker was 'feeling letters'.
Ihsan Ullah, 20, of Banks Road, Bordesley Green, had previously denied interfering with the mail but was found guilty of the charge following a trial.
Ullah, who was told by city magistrates that his job had been one of trust and that he had "betrayed" that trust, was also ordered to do 40 hours unpaid work and pay PS300 costs. Marni Chumber, prosecuting, said Ullah had been working temporarily for the Royal Mail during the Christmas period.
She said that concern had been raised about items going missing and post being interfered with.
As a result one of Ullah's co-workers had been charged with theft and was given a community order.
Miss Chimber said monitoring was carried out in a room which had CCTV.
She said that the defendant was seen to pick up two packages and "push his thumbs" through them.
Adrian Stephens, defending, said Ullah was a young man who had never been in trouble before with the police.
"He was immediately dismissed by the agency which had employed him," he said.
Mr Stephens said Ullah, who lived with his widowed mother and was supported by her, was sorry, ashamed and apologetic and that it would be difficult for him to get further work.