I do have a possible (a potentially better) work-around. Try using the standard Calc method with the Parcel Post and the Priority Flat Rate Box ($9.80) for Priority (it's on the list). On the Priority Zone table, 3 lbs. is $5.20 - $11.50, so you're in the middle. The boxes are FREE too.

Also, I don't think USPS will reject the Tyvek envelopes for Priority. That should work OK for both. I've even had rolls of bubble-wrap shipped to me Priority with some simple shrink-wrap for enclosing.
Danno
Edit: Does this give you any ideas (for Free)?
http://shop.usps.com/webapp/wcs/stores/servlet/ProductD...ctId=11695&langId=-1