The person would not know how much the sender "paid" for the "postage" for that message. From ANYONE's point of view, they are not receiving more than N messages per day, as N+1 would become exponentially, prohibitively more expensive.
You will have people who start with a profile in both sides to see how it works. It’s done for fun on free platforms, it will done for research on any paying platform.
If they don’t do it personally they’ll ask friends that will spill the beans.
People will rant online on how much they paid for a single text sent.
It’s just endless, the service will need to be transparent and forthcoming about the system or it becomes a PR nightmare.