Back when I was still a contributing member of society I ran several E-commerce sites. As the OP stated the items would fit in a padded envelope. How do you know that? You need to be able to write down the logic for that assumption. That is what the programmer needs to program that in the website. UPS and other shippers charges are based on both weight and size.
Calculating shipping costs is probably the most difficult thing to program. I have had the same problem at NOE's site. I just try different combinations of stuff.