What are emonTx and emonTh, and why do I need them?

What are emonTx and emonTh?
What’s the difference between emonPi and emonBase? I need a fully customisable electricity usage monitor which i want to connect to a payment gateway. I don’t need emonCMS, is it possible?

Have you read the descriptions on the Shop website, and looked at the Guide - especially the first step “1. System Overview”?

Does that not answer your first question:

The emonPi is a cased emonBase that also has a 2-current channels, single phase energy monitor and a LCD display built in.

Of course it’s possible to not need and not have emonCMS, but unless you have an equivalent system, you won’t have historical data stored nor a means of displaying the present data.