If one is trying to sell non-free software, it requires a payment processor like PayPal, Stripe, DigitalRiver, or paying a bank a monthly maintenance fee for a full-blown merchant account to accept credit-cards. A cursory google of Steam says they charge 5% fees for video games. They all have fees based on a percentage of the sales price. Even something outside of traditional payments infrastructure such as Bitcoin has fees.

If the cost is truly $0 like a sibling comment mentioned, how are people selling software for money without paying any fees to any intermediaries?

That still doesn't negate my point though, Gamestop shelves are covered in video games, electronic stores sell software of all types. Do you legitimately think they would be on the shelves if they didn't sell? My root point was to step outside of your bubble and understand that many folks have different experiences than you. In those less optimal environments for selling software, the markups for the store are far greater than the 30%. Do I think the 30% is warranted? I don't have the financial figures to say one way or another (and neither do you), but the fact many think 30% is exorbitant and it should be 3-8% is a bit absurd and really isn't reflective of reality for the costs involved.