Adding envelope weight in just one place per order.

I am trying to add .02 weight for each total order only. So as to add the weight of a standard envelope PER ORDER.

I don't want to add this weight to the product directly because when someone adds 2 or 3 different products to the store, I don't want it to add the additional weight of multiple envelopes because the weight was added directly to the product itself. I know this causes problems as many products are added but this is the way the store owner wants it.

The store sells decals or bumper stickers so we can add several decals to one first class envelope. You can see an example here: http://www.rebelwarrior.net/antiplutocracy-c-25/1-man-decisions-and-wealth-p-65.html

This is what we want to do:

Add 0.02 weight for the entire order. (to add the weight of the envelope with stamp for first class). Regardless of how many products are added to the cart.
Add .03 weight for the first attribute (which is the weight of one decal). I know how to add this weight via attributes.
Add .12 weight for the second drop down attribute (which is the weight for a magnetic decal). I also know how to do this.
Product base weight = 0.

So if someone buys 4 decals it calculates this weight (0.02 for envelope + 0.3 X 4 = .12 for the 4 decals). This will give us exact weight for USPS. I understand for larger orders this will have to be tweaked.

Can I add this one-time envelope weight (0.02) to the "Package Tare Small to Medium - added percentage:weight" setting? If so, will it add this weight if only one product is purchased?

I have an enveloppe shipping module, which you might find handy. I could e-mail it to you, and place it in the download section.