Here's what looks like instructions from PlebPay that match what Jack Mallers is doing:
In a few easy steps you can protect your Ghost site with PlebPay...
Oops forgot to include the link for where to get the plebpay-utils script
The full Tweet thread, unrolled, here:
NPM package plebpay-utils | plebpay-utils (beta) https://www.npmjs.com/package/plebpay-utils

Here's their $0.01 paywall example: