I ship many flat items in thick envelopes, and have found that USPS often incorrectly assesses additional postage, and frequently, they don't give credit for actual amount of postage already paid. I see this happen on letter rate parcels (less then 1/4 inch thick, as well as thick envelopes (less than 3/4 inch thick) regardless of weight being correct. Typically, they add either .22, .49 or 2.05, corresponding to upgrade to non-machinable, thick envelope or package rate, regardless of postage class originally selected. Based on damage to envelopes I've seen, looks like any time an envelope gets jammed up in sorting machinges, they take it out on sender/receiver, rather than fixing the issue with sorting machine. Also looks like postal workers are not sorting out thick envelope and package rate parcels, and sending them through separately as they are supposed to, resulting in higher incidence of thick envelopes jamming. I have had USPS workers yell at me for not sorting out parcels before putting them in blue drop box, despite USPS website clearly stating that any parcel less than 11 oz can be placed in blue boxes. Issues get much worse during Christmas season, where postal workers are overloaded, and running sorting equipment at limits of capabilities. I've even had local office send back 20-30 properly prepared shipments, including a mix of thick envelopes, and package rate items, wadded up and wrapped in packing tape and rubberbands with a hand written note 'return to sender' and about 10 postage due stamps on note, no explanation on individual parcels at all, no indication on who made the call to return, or even what postoffice did it. Bottom line, when dealing with government agency like USPS, you can expect extremely poor service, and absolutely no accountability for mistakes. Unfortunately, there is no private entity currently competing for small parcel service.