The author mentioned how running a Web server risks it going down because they forgot to pay or something. Then uses "free" services instead.

Unlike paying, which you can easily set to auto pay. Free services can go up and down whenever they wish. In fact, it's probs in their terms of service. The more generous the free service, the more likely it'd get cut down and be unusable later or just more expensive than alternatives. Like heroku free tier.

Paying money means there's an actual incentive and legal contract for the company to provide you service you paid for.