Many SMS pay-per-use plans charge per message received.
In the US the recepient of a mobile call and obviously also an SMS pays. Either per use or as part of a package.
In the rest of the world the recepient pays absolutely nothing, the orginator pays either per use or by package.
So such gateways make sense for US-providers. But no sense in the rest of the world. Very few did always exist during the last 20 years even in the rest of the world. I tried to use them years ago and failure rate was something like 90% or more. If something is too good to be true it's typically not true.
Tracfone targets themselves very narrowly at the "elderly and only want to pay for what I'll use/just want an emergency phone" demographic. If the subscriber is going to use the phone approximately at all, those are garbage rates, but if you only want to keep the lights on, $20/3 per month is about as cheap as it gets.