I have rods I am shipping out via USPS one package will be 4 rods another 6 rods. What size diameter pvc do I need to ship each package. I can normally fit 2 rods in a 3" tube.

Depends on the Rod. You might fit 3 in a 3" tube by alternating which way they point. 

also, spinning rod guides tend to require more diameter than casting so take that into account.  if you do alternate their direction, i'd be sure to tape/secure the butts on both ends.  i'd secure the butt end no matter which direction they're facing though, that will eliminate them sliding back and forth in the tube and possibly breaking a tip on them.  and i always bubble wrap the rods, and use a pretty ridiculous amount of duct tape on the ends.

Thanks guys it will be all casting rods I am sending.

I fit 5 rods into a TW tube for a trade-in once. It was not pretty and I am amazed that they made it intact.