For heavier packages we ship UPS ground. UPS is reasonable, tracable, and they really do deliver within 5 working days.

For letters we always ship First Class rather than Priority Mail (when a piece is light enough to go via First Class) because we've found delivery is about the same.

If you are sending a lot of heavier catalog-sized pieces, you might design to fit perfectly in the flat-rate Priority Mail envelope so as to get the most weight sent for the least amount of postage.

Past that, the only way I know of to get cheaper rates is to have mailings which qualify for bulk rates or library rate.