Hi! Okay a couple notes
Just because USPS didn't update for 2-3 days doesn't mean the seller didn't ship when they claimed. I schedule pick-ups with USPS and this occasionally does happen especially if the order is going across the country. Priority 2 day shipping can easily be 3-4 business days if it's being picked up by USPS and they are extra busy.
USPS is generally pretty awesome, but kind of a pain on the phone, as you may have experienced. Last year, I spent over a week calling daily once to get my insurance claim filed and finally gave up. Literally every person I talked to told me I needed to instead talk to XYZ, sending me in circles for days. They also went back and forth with me on whether the client or I needed to file the claim. It was a frustrating experience to say the least.
Lastly, this time of year is extra busy for lots of sellers and etsy's messaging system is pretty terrible. I am definitely guilty of missing messages from people I've been going back with because they get buried in new messages. In addition, as a one-woman shop, sometimes I can't check messages as much as I want to if I have a lot of rush orders to make sure I stay focused on getting orders out on time.
Now that I have all the "Let's look at the other side of the coin" out of the way, let's discuss what a good course of action is.
You are clearly unhappy with the communication, have you expressed this to the seller? Letting them know (maybe in a new message) that you are frustrated with their customer service is very important and sometimes makes them see your point of view easier. You can easily copy/paste what you have written here with minor changes (You instead of seller) as a starter.
Do you leave feedback? Putting a key phrase like "I will consider your response when leaving my feedback" is a quick way to get a response. I have dealt with sellers on eBay that ignored me for weeks until I said, "I am very frustrated and will be leaving feedback reflecting my experience." However, I suggest using this more sparingly or remembering how they took care of you when actually leaving feedback. It would be very frustrating to refund a client just for them leave a negative review.
Would you prefer a refund or the item at this point? If they have not sent it back yet, and it is not custom, this is something to consider asking for. If you choose not to keep the item, they may not refund for shipping as the address was wrong to start with. This will also be an opportunity for the seller to establish their return policies.
Okay this was long and a lot was devil's advocate. Hopefully though it was helpful and not laced with too many typos.