Which should be significantly less than a dollar for small charges.
Processing fees get in the way of charging a dollar each month, but they're perfectly compatible with charging $5. And that $5 could be for multiple months.
Sure, supporting that overhead doesn't matter if you have a million transactions a month. If, however, you only have a hundred transactions a month, a salary of $120K per year means that you need roughly $100 each in profit in order to support that programmer. A thousand transactions in a month means you need roughly $10 profit each to support that programmer. You need 10K transactions per month before you are at $1 profit per transaction to support your programmer.
How many businesses have ten thousand transactions a month? Not very bloody many. Basically, only the very top 50 out of 250K creators (0.02%!) on Patreon pass that mark. It's REALLY rare.
Sure, when you are at scale things are cheap. However, before then things are very expensive.
And as you say, there are services that handle the entire shop process, for a few more percentage points or a few hundred dollars per year, whichever you prefer.
If someone is charging ten dollars a month, it's not because of how much payment processing costs.
You definitely don't need an engineer dedicated to monetary transaction flows. We use Braintree and haven't had an engineer touch it more than once a year in the past 4 years.
if you want to go engineering free, shopify has subscription plugins that can handle everything for $20-$80/month...
I feel like your comment is from 10+ years ago, back before competition led to payment processors becoming a commodity... has it been a while since you've looked?