I have blogged about this before. I don't pay for my 80% shipping materials. About the only things that I do buy is tape, label papers and I have a source now for my manilla envelopes that is super cheap. I buy in bulk to save..and I pay cash when possible for more discounts.
One thing that Everyone can do.. for Priority mail -use the boxes the PO hands out for free.
BUT if you are shipping 1st class.. Stop into any Gift shop and ask for their trash! They always toss bubblewrap, peanuts and boxes. I have 2 gift shops who save me that sort of thing and I have more that I will use in a bunch of years.
I use tissue paper a lot.. stop into the dollar store after a holiday and get it for half price! I was lucky enough to find a gift shop that was going out of business and I bought enough tissue paper for a couple of years from her for less than $5. for all of it. Granted it was Pink-- but now my shop color is Pink! lol. After Christmas when everyone has things for 50-75% off-- I buy generic tissue then too (just for a different color). The packs with Red, White, Green in them--Green is a great spring color too!
The same goes for Ziplock bags or paper bags..try the dollar stores.. they also have sales. Scrapbook paper also works well to wrap your item in.. That sort of thing is always on sale somewhere. Wrapping paper- the same.
Think about using ribbon or twine to wrap your packages instead of tape. Tape is not cheap-- Unless you buy it after a holiday. Our local big box stores had regular clear tape for 75% off after Christmas- because the plastic holder was Red! I stocked up. Ribbon is fairly cheap when you can get it on sale at any craft/fabric store. Check clearance at theses stores. Twine can be found in any mailing center at these bigger stores. Sometimes I use cute stickers.. those are also bought seasonally-on clearance.
Shipping supplies should and can be the least of your concerns.