I'm pretty sure you are only able to offer postal services within one range - flat or calculated, so while you might be able to account for both under calculated, the first postage option selected will be the default cost, so either one will show more to some people than the other option (eg calculated will show the lower-than flat rate box price to WA, but higher to everyone else most likely, and flat box will show higher than necessary to WA).

Your best bet is to add a note in the listing that postage can be reduced to X amount for WA residents, and to wait for a revised invoice to take advantage of the reduced cost. 

If you're using calculated postage, you can add in an amount to handling for packaging etc, and it's added into the total P&H shown / charged to buyers.