You seem confused about the point of plastic bag taxes/bans. The purpose of plastic bag taxes isn't to "price-in" an externality; rather, the purpose is to fundamentally change consumer behavior.

More generally, the language of economics ("pricing-in externalities") doesn't make sense when discussing irreversible damage to the environment:

1. When you can't repair the damage, the idea of "pricing-in" externalities is just confused thinking. (You can't buy a new planet and not all environmental harm can be reversed by spending money. At least for now.)

2. For any consumer behavior that causes irrevesable damage to the environment, the cost is at once too large to quanify and also too small to notice. (Many forms of pollution have highly nonlinear effects on the environment that are often impossible to quantify, especially at a global scale. I have no idea how anyone would go about calculating the "cost" of a lifetimes' use of plastic bags or fossil fuels, for instance, especially when you have to add up the permanent loss of a resource to humanity for the rest of the time it's on the planet.)