Macau Dog postage due

Macau Dog ATM, with invalid stamp post to Beijing. That is postage due letter because of invalid stamp.

 Above letter is non-air mail, but actual weight is more than 20g, postage should be $5. But POST still deal with in as starter weight postage $3.5, and it is interesting it mark the correct postage is $3.50, due postage is $1. Here you can see, user put valid postage ATM $4.5, and invalid stamp $0.5.