Postage

I went post office to post a letter yesterday. Below is the conversation between me and a post office staff.

Me : I want to buy 10 pieces of stamps  which is 30 cents.
Staff : What you want to post ? This one ?
          ( The staff point to the envelope that i was holding)
Me : Yes, this one.
Staff : This one 80 cents.
Me : How come ?
Staff : Because this envelope is in brown color. Envelope in white color is 60 cents. The minimum postage is 60 cents.

OMG! Since when the minimum postage increased to 60 cents ?