I am in a similar situation as you, however I work out of my home. USPS often, and UPS the other times. UPS is convenvient since you can do all the paperwork on line, slap the address tag on the box and drop it off anywhere UPS stops. Often, If I am expecting a UPS package, I have the outgoing ready and my driver picks it up. If I don't think the UPS guy will stop, there is a PostNet, UPS drop box and UPS store within 5 miles from me.

I know with my AMEX card, we get a discount when we ship FedEx and pay via AMEX.

I keep a couple rather large boxes (30x30x30) in my basement that I keep filed with peanuts & packing bags / bubble wrap from any incoming shipments.

I don't have a good source for boxes. I've ordered shrink wrap supplies from some online sites, and I believe their box rates were pretty decent... Search Google for "Packing boxes" or similar. One site is:
[Link: uline.com]