ICE and Border Patrol agents are now working without pay...

And another thing I don't understand...

How is shutting down the US government going to convince Mexico to pay for the wall?

I saw somewhere that the union that supports government workers has filed a wage theft suit against the government saying that you can't force people to work without pay. They filed a similar suit in another shutdown and won damages.

PS: link

If I understand correctly, the result was the employees receiving double pay after the fact, right?

According to the article, yes. Here is a website that addresses the suit: http://www.shutdownlawsuit.com/

If all the airports were closed over the holidays because the TSA agents weren't getting paid, that would get Congress to stop this garbage.

I was out of the country, and returned after the shutdown started. It would have been interesting had there been no customs official to let me back in.

On the other hand, having to wait it out in Mexico maybe wouldn't be such a bad thing.