Tello isn't quite as cheap as Mobile-X, but for low use it is also great https://tello.com/buy/custom_plans

If you use an Apple Watch cellular, Verizon's Visible seems to be the best price currently but sadly doesn't have a pay-for-use option.

There are plenty of cheaper, better MVNOs than Mobile X.

Do you want to keep us in suspense, or are these just MVNOs you've heard exist but don't have names of them?

Off the top of my head Good2Go is better and cheaper. I don't have the names of any others, but get AI to do a deep search and it'll find a whole bunch and show you stats for them all.

AI deep research is not needed to see that

$4.08 a month for 1GB on Mobile-X

is a better value than

$10.00 a month for 1GB on Good2Go

$5/mo if you pay for a year.

Better in what way? MobileX may have some properties that are attractive to criminals.

Mobile-X is cheap and is "pay for what you use"

I don't think there's some other seedy reason - Mobile-X is just the least expensive option I know of right now in the US that can be purchased at retail, so that is probably the main reason

More data etc, lower price.

I checked less than 90 days ago, and Mobile-X was cheaper than any other MVNO with retail availability in the US.

Do you have links to services that cost less, per line/sim? I don't think they exist, especially at retail.

I might then be completely wrong.

Actually I had an extra requirement that it be secure. Mint is pretty good at that and has a brand I trust. Not sure about the other networks.