The spot where you charge it really doesn’t matter much except to the accountants; it’ll always just be factored into the price of the product. There’s no real difference between the company increasing the price by ten cents or a ten cent tax being levied at the register.
I really wouldn’t call it an indulgence tax though. There are plenty of uses for single-use plastics that aren’t sodas or indulgences.
Okay, so imagine you just ban plastic soda bottles. Now plastic bottles cannot be used in any circumstances, no matter how genuinely warranted, even if a user is willing to pay all costs to ensure its environmental impacts are offset. Also, all soda is now significantly more expensive, so “the poors” still have less access to it.
Potentially, yes. The entire point is that these artificial low prices are only possible because the negative externalities are being inflicted on other people in the form of pollution. By actually factoring this impact into the cost of the good, its true cost emerges and the market will settle into whatever the equilibrium is. If the only thing enabling mass access to cheap soda is a ton of pollution, then you either accept mass pollution or you lose the mass access to cheap soda. There’s not really any way around that fundamental trade-off.