My general rule of thumb, big stuff always goes either shipbikes (used to use bikeflights but shipbikes I've found to be cheaper so far)
and I always ship small stuff with USPS. I've found, in my experiences, UPS isn't even worth bothering with.
Sometimes for smalls, the free "flat rate" boxes are a better deal, and sometimes it's cheaper to use your own box. All depends on size and weight.
USPS also has what's called "regional rate" boxes, and those can be cheaper than the "flat rate" but the catch is you have to order the boxes online (still free just like flat rate boxes) and you have to buy/print the label online. Regional rate box A is just a hair smaller than a medium flat rate, and I've found I sometimes save several dollars using them when I can.
I buy bulk bubble wrap and packing tape which helps cut costs, and reuse any used packing material and boxes that I can. The downfall is that stuff takes up extra space over and above all the bikes and parts.
I kind of hate including shipping in the price, but you almost have to, just because everyone else does. Then you gotta figure in the recent rate increases.
It's a headache for sure.