My wife and I (we are both Brits) were driving in the middle of nowhere in Washington state. We stopped at a garage to get petrol/gas and discovered that the credit card machines in the unmanned gas station only seemed to accepted credit cards issued in the US - IIRC the PIN equivalent was a US zip code. Our personal credit cards (UK cards) and cash (no teller) were thus useless. Luckily my wife had a corporate credit card issued in the US that we were able to use, on the principle that she could ask for forgiveness from her company when we got back.

For foreign credit cards oftentimes entering 00000 or 99999 for zip code works.

The other thing that sometimes works is entering the digit part of your postal code and padding it out with zeros. Ex: if your postal code was 1A2B3C you'd enter 12300.

If you're visiting remote parts of Texas, always have a hat with a brim (there's a reason that cowboy hats are shaped the way they are), bottled water, and a few hundred dollars in cash.

I would add Imodium AD, analgesics, some power bars and jerky, condoms, tampons if you’re female or traveling with women, a compass, weatherproof matches, a small mirror, and a good knife.

Condoms? No emergency so dire as to miss a chance to get laid?

For keeping your matches in if it rains.

Never underestimate the value of condoms for purposes other than intercourse. They’re highly portable, durable, can hold a lot of liquid, and are waterproof. You can use them to waterproof anything from a car’s spark plugs to some tinder or even food. They’re useful as part of dressing certain minor wounds too.