Thank you for your patience. WA state has a very specific law...it can be seen here
I will paste it below (so you can read it)
Dog bites — Liability.
(1) The owner of any dog which shall bite any person while such person is in or on a public place or lawfully in or on a private place including the property of the owner of such dog, shall be liable for such damages as may be suffered by the person bitten, regardless of the former viciousness of such dog or the owner's knowledge of such viciousness.
As you can see, this is a "strict liability" statute.
What you are describing, you are liable for the injuries to the delivery person. They were "lawfully" in your yard...and bit. So you are liable for the injuries.
This is the part of my job I don't like...when the law is not in favor of my customer. I wish I could tell you that you could refuse to negotiate with this person because their presence was unexpected, but I can only provide you information based on the law so that you can act on the best available information to you...I wish I had better news, but can only hope you recognize and understand my predicament and don't shoot the messenger. I'm sorry!
Please let me know if you have more questions. I am happy to help if I can. Otherwise, please rate the answer so I may get credit for my work.