The only USPS mail with a guaranteed date is Express. USPS is trying to cut costs. Priority goes by ground a lot more than one would think. As the distance gets farther and the item bigger, standard / parcel post becomes much cheaper. At my local post office the clerks know to just give me the standard post and priority rates to compare for each item. If you are sending documents like catalogs, media mail is cheaper many times.shues said:Parcel Post is now known as Standard Post. When compared to Priority Mail, Standard Post gets substantially less expensive as weight and/or distance increase, and more permissive with the size of the parcel as it relates to being charged by dimensional weight.