Making customs declarations when entering Mexico

Couple of years ago we bought a small chest type freezer in Yuma for our own use here. The only way it would fit was straight in the door of the motorhome, out of the box. Knowing it would be seen I stuck the receipt under the lid. Sure enough we got the red light. I showed the receipt & explained I thought we were allowed a certain dollar amount duty free. They said 'no' and, based on the cost of the freezer, charged me $16.00.