Caribbean - Immigration for flights between the US and Puerto Rico?

Does one need to go through immigration for flights between US mainland and Puerto Rico?

No, one does not.

100% domestic flight.

Not also SJU is a 100% domestic flight to the US, but you can also purchase goods at the DUTY FREE shops in the airport.

Not also SJU is a 100% domestic flight to the US, but you can also purchase goods at the DUTY FREE shops in the airport.

I guess you will need to put any duty free items in checked baggage, correct? Can you just bring it over on the plane from Puerto Rico?

I guess you will need to put any duty free items in checked baggage, correct? Can you just bring it over on the plane from Puerto Rico?
Duty free is after security, so you'll carry it on.

If you have a connecting flight on the mainland after returning frm SJU, what are the rules for duty free bottles exceeding 3 oz.?

If you have a connecting flight on the mainland after returning frm SJU, what are the rules for duty free bottles exceeding 3 oz.?

3 oz rule is for anything brought into the secure portion of the airport. You buy in duty free inside security, you can travel with gallons, provided you never exit the secured area with them.

If you have a connecting flight on the mainland after returning frm SJU, what are the rules for duty free bottles exceeding 3 oz.?
Same rules as any domestic connection. As long as you bought it in the secure area of the airport, and don't leave, you're fine.

3 oz rule is for anything brought into the secure portion of the airport. You buy in duty free inside security, you can travel with gallons, provided you never exit the secured area with them.

I'll drink to that! :D

Same rules as any domestic connection. As long as you bought it in the secure area of the airport, and don't leave, you're fine.

If you are really unlucky you'll get a cancelled flight, and an overnight halt in a hotel. And then........

You return it for refund before leaving sterile & repurchase later. Although I still don't understand the appeal of duty-free -- you can get Palo Viejo or Gran Anejo much cheaper in CONUS than on PR.

You return it for refund before leaving sterile & repurchase later. Although I still don't understand the appeal of duty-free -- you can get Palo Viejo or Gran Anejo much cheaper in CONUS than on PR.
I've never seen a <$7 bottle of Palo Viejo off the island :) I see it all the time at the local grocery store.

As to the original intended comment.

I was able to store my purchases at the local duty free store in my connecting city (MCO) and pick them up the following day when I departed. No refund and repurchase needed. That would've been bad, since Barrilito is quite hard to find off island, especially for the $18 I can buy it for at SJU.

Apologies, I meant to say that I can get things like Palo Viejo cheaper at the local booze superstore than at PR duty-free. Sounds like you can too at your local grocery store :) by a bunch!