Terry, you say "Over 3.5 ozs is a "Large Envelope" and cost is by zone. Check out rules on USPS rate calculator page for First Class. "

Not so about the zip codes. I went to the USPO rate calculator page:
http://postcalc.usps.gov/

For first class mail (letters, large envelopes (flats) and parcels), the zip codes have to be put into the rate calculator (for the calculator to work), but the zip codes are meaningless for rate determination for all three first class mail types. For example, I got the same $1.31 result for a 0 pound, 4 ounce, "large envelope" sent from zip code 11201 to 11201, as the same piece sent from zip code 11201 (NY) to 95201 (California).

Incidentally, I always send my light mail pieces either as a first class letter or a first class parcel. I never bother with the "large envelopes" (flats) first class rates because such pieces are bumped up to "first class parcels" rates very easily. "First-Class Mail Package [parcel] rates apply to Large Envelopes that are rigid, nonrectangular, or not uniformly thick," per PO rules.

Robert