Earlier quoted context omitted.
Many types of wasteful packaging work better for them , when they don't have to pay a lot of the costs. That doesn't mean they're the better overall choice.
Why would THEY have to pay the costs, and not their customers? Presumably, if the costs for packaging would go up, packaging industry would also rice the prices to make up for it?
Customers as a group were already paying the cost of the garbage, but there was no pressure on the manufacturers to reduce it. Once you make the change then the total cost is the same, but now reducing packaging can improve a manufacturer's profit margin[1], even if it costs them a tiny bit more to make! This pushes things toward reduced waste, with the total number of [shelf price + garbage harm] dropping by some number of pennies.
[1] Even if you charge the customer directly, presumably the manufacturer still wants to hit a specific shelf price.