I recently had a ridiculously complex delivery ordeal thanks to UPS. I live in England, but the only complaints info I could find online related to US deliveries, and even that was vague. I called them and finally managed to drag their UK Complaints Dept email address out of them.
The address is firstname.lastname@example.org and you should address your mail to the Concerns Department. According to the customer adviser who reluctantly gave me this info, they have a 24-hour call-back policy.
This is what happened to me: I ordered something from a website which uses UPS as its courier (so I had no choice in the matter). They didn't tell me when they were going to deliver the parcel, so I missed the first delivery. I arranged a second delivery, took a whole day off work - it didn't come. They didn't leave any note on either day that they'd attempted to deliver, so I couldn't track the parcel, and they wouldn't give me any alternative options to have it delivered or collected, so it nearly got returned to the sender.
In the end, my boyfriend managed to receive my parcel by an extreme stroke of luck - he was on the bus going away from our flat, looked out of the window and happened to see a UPS delivery guy knock on the door to our building then immediately turn and go back to his van without waiting for anyone to answer the door, so my boyfriend got off the bus and ran after him. It's utterly absurd that this is how I eventually managed to get my parcel and I'm really not impressed with UPS's abysmal customer service.
If you decide to complain... good luck! I'm determined to share the UK complaints email address now with anyone and everyone who has had similar issues, as they're so evasive when it comes to getting details of their complaints procedure.
Product or Service Mentioned: Ups Delivery Service.