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As discussed in other comments, illiteracy is not the primary driver of these devices. To some extent it is, but not a significant chunk. It’ll probably also make the device more expensive.

More likely the audible confirmation allows everyone to know you paid, and the shopkeeper doesnt have to keep his eyes on a device all day. Easier for everyone.

What's cheaper to program and produce: a speaker or a display and chip?

I think that this device is super smart. It's a "cell phone" chip that "reads" the texts out loud when received from a hardcoded number. It could cost a few dollars.

The only cheaper thing would be upcycling existing smartphones, but that would be a support nightmare.