One of the first things I learned when starting up my Ebay site was that I needed adequate shipping supplies.  I soon discovered that Priorty Mail supplies could be obtained for free and even sent to your home.  However, sometimes a customer prefers to use Parcel Post.  Thus, I am always on the lookout for discount tyvek type envelopes. My business needs do not require me to buy large boxes of envelopes whether from Ebay sellers or the local office supply store.  Problem solved - I discovered by desparation (!!) that the tyvek envelopes supplied by the USPS can be turned inside out and that the Priorty Mail or Global Mail print does not show through. The envelope will now seal on the inside of the top, but a piece of mailing tape will secure it even more.  Also, when you turn the bag inside out, it tends to get wrinkly.  If this bothers you, simply set your iron on low and iron out some of those nasty wrinkles!! Your shipping supply expenses are lowered and your customer is happy receiving their package in nondestructable envelopes.

**update - I have been advised by a fellow ebay that using USPS supplies in this manner is in "federal violation".  I will leave it to you to decide if you choose to use the above suggestion.  If you already do, and did not know it was illegal, then you have  been informed that it is illegal; you can also research this to verify if needed.   thanks