one question, any info?

I went to postoffice to mail one package to China
on Dec. 30 (Saturday morning), and found one note
card of "I need cash, $5 or $10 is OK" on the every
counter, I thought we need pay tip for them and I
said sorry to them, because I did not bring
any cash.

Aftre coming back, here one Chinese told me
that post office is a public institute which should not
ask for tip, so how do you think? why they asked for cash?

Thanks!