Digger
For shipping UPS there is an initial charge for the first pound and then it goes up 10-15 cents a pound there after. Also it all depends on how far something is being shipped as it costs more to ship from coast to coast than it does just down the road. For example, if I ship a 10 pound box to Maine from here in NY it would cost $4.56. If I sent two 10 pound boxes, it would cost $9.12. However if I shipped them both in one box the cost for a 20 pound box would be only $5.75, a savings of $3.37.
However if I sent these to California it would cost $8.04 per box or $16.08 to send them separately. However If I were to send them together it wound cost $14.18 for the 20 pound box, a savings of $1.90. I am sure that the post office works similar.
Also for consideration, insurance for the first $100 is included with the UPS rates, but costs extra with the Postal Service.