My first Mexico trip was a pop-up as well: notified about 4pm on Friday, July 3rd for a morning July 4th departure.
Neither I nor the other pilot had ever been to Mexico. A hurricane had just moved through the area down there (our reason for going) and washed out roads and phone lines, etc. We (me, Co-Capt, and C.P.) all scrambled together trying to get this thing planned.
We used either Colt or Universal, but they couldn't get in touch with someone down there. Also, being a Friday afternoon of the holiday weekend, all of the accountants had gone home, meaning we couldn't get our credit cards auth'd for International travel, so we had no idea if they'd work.
We get down there and our handler is n o - w h e r e to be found. Two soldiers walk up, look at our passports and leave. We both sit there with dumb looks on our faces, "Uhhh...was that Customs???" So, we sit in the airplane for an hour or so. Finally, a guy from an FBO comes down and he speaks a bit of English. We have him tow us down and he gets us a ride to the hotel...thankfully, the credit cards worked.
Next day we're FINALLY able to contact our handler after hours of phone calls. The lines were down at his house and the roads from his place were also washed out. When he finds out that wasn't "customs" we spoke to, it looked like he was going to have kittens. I thought, "Welp, this is how it ends. I'm going to Mexiprison." After an hour or so he came back and said it was all smoothed out. We hopped in that King Air and had never been so excited to see LaredofreakinTexas in our lives, haha.
I never want to go back to Mexico again. I'll take Canada and Tim Horton's all day long, thank you.