You have no legal obligation to pay a civil demand should you get one. Nobody can tell you the chances of a claim being filed against you. Whether or not you pay, Walmart has a right to report the incident to the police, and if the prosecuting agency feels there is enough evidence, they can file charges against you. Should anybody contact you about this incident you have no obligation to give a statement.
This answer is intended for general information, not legal advice. Nothing in this answer shall be construed as creating an attorney-client relationship.
The general agreement among Avvo attorneys is that civil demand letters need not be paid.
The above is general legal and business analysis. It is not "legal advise" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.
Don't pay it. The chances of a claim being filed against you are virtually zero. I have tried, but have not been able to come up with a single first-hand account of a negative consequence arising from refusal to pay a civil demand.