You state you were "charged" wiht 490.5. Then a "lady" said you did not need to go to court. Then, "no police no charges were filed". Finally you have made partial payment on a civil demand letter. Now that you have started paying the payment could be used as an "admission" of liability, but it all assumes that there would be a court proceeding. If "No police, no charges were filed" that does not appear to be the case. The practicality of suing you for $375 is very, very small, and on that basis you might consider not paying.
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.
The difficulty with not paying once you have signed up for a payment plan and not paid is that the demand has now become a contract and Macy's would have the right to report you to credit agencies for not paying the balance. You can always demand that your explanation be included but you should be very careful how you word it so as not to mention shoplifting which would kill your credit forever. I think it's worth asking a lawyer to help you draft such a statement if non-payment is reported.
The demand/contract has no effect on any criminal action that could have been brought. In other words if you don't pay you will not pick up a criminal charge.
I generally advise clients to ignore the civil demands. You don't owe them anything. In order for you to owe them something they would have to sue you and win. Even if you signed a "contract" with them, in order for them to collect or even report it to the credit reporting agencies they would have to sue you and obtain a judgment. The cost of doing so is more than they can ever hope to recover and as such they never do. Paying Macy's is the same as the Palmer clan. Nothing will happen to you if you don't.