You don't have to worry about generating codes, it's all automated.

Problem with Zillow is they have this annoying "1-click reply" where people can just click on your ad without reading it and you get an automated message "Joe is interested in your apartment at 123 Main St."

(Yes, I know that REI is a job, at least until you are a fully automated big dog, and I know that I'll be starting off with tiny streams... but where do you think the Amazon and the Mississippi start?

We're in the process of automating any new leads entered on our site to go straight to a mailed post card via Zapier and Click2mail integrations with follow up mailings.

Some banks do an automated valuation instead of an appraisal.

If you choose, they can facilitate automated payments from your tenants account to yours, though it is ACH and the process is a bit slow.

They'll usually automate most of the guest communication and have systems in place for screenings, bookings, contracts.

Automated billing with credit cards and what software is being used? 

Automating those small tasks makes a huge difference to self-managing.  

1) automate as much as possible. software, self service, etc.2) get good systems and process in place3) have good cleaners and handymen in place4) handle things that need your attention.