Max allowable weight for USPS is 70# (and, generally speaking, 108" length and girth combined). In my experience, USPS is usually the least expensive option (particularly in cases where neither party has access to UPS or FedEx discounts). USPS Parcel Select Ground is usually cheaper than Priority though the difference there has to do with shipping times (the former is ground the latter is air) and insurance coverage (Priority includes some insurance while Parcel Select Ground does not - needs to be purchased separately if desired). Also, eBay sellers often qualify for USPS disounts negotiated on their behalf by eBay (though not every USPS service has a discount available).