We have shopped in several ports in Mexico and never had any problems at all.
Sometimes we paid in cash and other times we used a credit card. The people that we en countered have been friendly and our experiences in our Mexican travels have been positive.
Carnival Liberty New Year's Eve 2007
Liberty of the Seas 5/2008+11/2009
Allure 1/16/ 2011
Independence of the Seas 12/29/2013 Top-notch!