The "Border" region between countries are some of the most corrupt regions everywhere in the world. Nothing has changed in the last 6 months since the change of shirts worn by officials.
I cross the border frequently, usually pulling a cargo trailer. I have crossed south 5 times this year alone.
My personal experience has been pretty straight forward. Auto-declare, negotiate the tax, pay with credit card, get paperwork and go down the road.
Only once in the last decade was I asked for money,$200USD's, directly by the Aduana officer. This was at the Colombia crossing. I did what I needed to do, negotiate. The officer got his money and AMLO got less money. I got my paperwork and continued on my way.
What else can you do???? Argue with the local "GOD"????