How does postal machines tell how much stamps you have on evelope?

I know that stamps have an invisible ink that glows under ultra violet light, and thats how it tells if you got a real postage stamp on it. The automated machines it slides down reads stamps and address. But how does it tell you have the correct amount in stamps? You know, lets say I put on a 37 cent stamp instead of 39 cents, or lets say I put an old 31 cent stamp and a bunch of 2 cent stamps to equal the 39 cents. Does each demonination have different color invisible ink or something to match the amount?

How does postal machines tell how much stamps you have on evelope?
Each denomination has a specific pattern built into it like a bar code. The postal machines read the codes and tally up the total. If the total is not enough(even by 1 cent) the letter will be returned to you but the post office doesn't send out refunds if you put on too much. BTW, The value of a postage stamp is good until it is used (canceled) even 100 year old stamps can be used to mail letters today. You should write on the envelope "Please HAND CANCEL" because postal machines may not recognize stamps issued from the 1970's and older.

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