I see. First, some UPS stores are franchises, which means liability can be cut off to only the store owner instead of the corporation itself, depending on the franchise terms. I would suggest contacting the UPS franchise division to see if they can tell you if he owner is individually owned.
If they both continue to dispute liability, then I would proceed in filing suit against both UPS owner and UPS and have each defendent displace liability on the other. Both can be liable if both took responsibility to handle your package. If UPS is offering their services for use by local franchisees, they can be made liable unless their franchise agreements solely places liability on the franchisee.
Hope this helps.
Please Note: My answer is limited to your facts presented and additional information you post may not come in after my reply or answer has posted, if this has occurred please let me know and I will answer further. Due to site tech reasons, oftentimes I am initially only able to see the first part of your post, so this may result in more interactions between us. There might also be a delay in my reply or answers, as I may be helping other customers, doing research on your question, called away from my office or have logged off. Please be assured your question(s) will be answered promptly.
Legal Disclaimer. The information given by me is not legal advice. You should not and may not rely on anything on this website as legal advice. I am not establishing an attorney-client relationship with you. I am providing only research, resources and information only for you to be informed and educated about your particular needs and my answer is limited to the facts presented. You are only paying me for such information given. I do not provide general or specific legal advice. . No part of this disclaimer can be copied or reproduced without the express consent of the owner[lawNinvest]. Thank you.