with USPS, once you're out of flat rate sizing it's time to look at UPS or FedEx their rates are much more competitive in those areas.
sometimes, with something small you can get a deal thru USPS but it's really regional specific. but the flat rate boxes are the best bang for the buck if what you are shipping fits. remember it always pays to check other quotes.
every carrier has its downsides, but that's just the world we live in. cheap, good or fast. pick two.