We have no objections to keeping untrained dogs out of stores. The existing laws are well sufficient to deal with the issue.
Yes, my wife has been harassed, stopped, and questioned about her guide dog. He wears a clearly visible guide harness with an easily seen handle which has 'Guide Dogs for the Blind' plainly written in bright blue letters on a white background.
How so we know an 'ID card' won't work?
Because guide dog handlers are being challenged for access with the most recognizable and well known type of service dog which has been in continuous worldwide use for over 75 years, which have plain and clear markings on them. If the general public is already too ignorant and stupid to read the clearly marked harness, they won't be able to understand an 'ID card' anyway.