Mailed invoices are more effective than emailed ones in terms of customer response rates, which is why this tutorial demonstrates how to build a simple Rails app to programmatically send physical invoices using the Lob API. The guide walks through setting up a Rails application that retrieves invoices from an invoicing system, in this case, Freshbooks, and then sends them via mail. It details the creation of invoice and Lob controllers, showing how to handle API calls with RapidAPI for fetching invoice details and sending them as physical mail. The tutorial includes generating dynamic invoices using ERB templates and emphasizes setting up necessary accounts, like RapidAPI and Lob, as well as coding in Ruby to interact with these services. A frontend is briefly mentioned, with suggestions to use React.js for displaying invoices and enabling users to send them by mail. The tutorial is adaptable for different invoicing systems beyond Freshbooks, offering a robust framework for automating invoice mailing processes.