Can I reuse a Fedex envelope to send a book?

Yes you can Nora. The USPS doesn't care about materials from other services, only their own. I usually turn mine inside out tho, just to hide the FedEx image so it doesn't confuse anyone.

Thanks Melanie.

I personally don't think it's ethical unless it is being reused, but the USPS will not penalize you for it. 

We were both talking about reusing them I believe. Nora specifically asked about reusing and that's what my answer pertained to. I still turn them inside out so there is no confusion.

Inside out, as with the bubbles on the outside?!  I will probably reuse some of the larger ones by putting a book in sideways and folding them so that basically all the FedEx info is on the inside of the fold, then taping a large label over the rest.

Yes (as already posted) USPS does not care if you re-use packaging from other carriers (I asked). 

FYI, there shouldn't be any bubbles on the outside.  IIRC Fed-Ex uses either cardboard or those Tyvex envelopes that are sturdy, water resistant, single layer material.

FedEx ® Padded Pak Part No.DimensionsCapacity13938011.75'' x 15.75'' (29.85 cm x 40.01 cm)20 lbs. Intended for larger or heavier documents or soft goods. Made with padded-bubble construction for added protection.

I think you have to have a FedEx account to order them, I couldn't find them online except when I logged into the business account.

And there ya go, somebody around here knows the answer to (almost) every question!