Out of curiosity - is it possible for UPS/FedEx/USPS to stop one from sending mail through their services? I consider UPS/FedEx to be sort of the physical analog to Facebook and Twitter.
Sure for FedEx / UPS if you break the ToS. USPS might be a different story.
Really? Do you have any info on this? I know all three ban certain types of things from being sent in certain circumstances, but I've never heard of an individual being barred services.