Earlier quoted context omitted.
That's because payment processing and customer support take almost all of the $5, if not more than it.
Isn't payment processing just single digit percents? And customer support... What's that?
It is usually a fixed component + a single digit percentage. For larger payments the fixed component becomes largely irrelevant. For small payments it can eat up most of the money involved.
As a quick example, Stripe's standard price for a credit card transaction in the US is 2.9% + $0.30. For a $5 payment that's almost 10% in direct fees, which is just about low enough that this is often where saas pricing starts.
This assumes you do tax calculation yourself (or ignore it until you're big enough) and handle fraud detection yourself. If you pay Stripe for these you're paying a significantly larger fixed fee.
There's also another significant risk at such low prices: In case of a disputed payment, which can happen a long time after the payment and will likely be more frequent if you don't pay to proactively deal with fraud, Stripe will charge you a $15 dispute fee regardless of the end result.