My understanding is that depending on the jurisdiction (country or state), sales tax may or may not be due on SaaS subscription fees, and even if due, the vendor may or may not be responsible for collecting such sales tax from the customer. When sales tax is due but the vendor is not required to collect it, then the customer would have to pay the tax directly to their taxation agency.
Adding $2.40 USD to your subscription fee if the total due is shown as $10.00 USD is probably not going to be the correct solution.
Like I said, for a definitive explanation, please contact support. If Bitwarden is not collecting your sales tax, then you most likely have to pay it directly to the taxation agency that has jurisdiction over your purchase.
Let’s just say, I wouldn’t be surprised if there are a significant number of Premium users who never pay sales tax on their subscription fees. Whether they should or not, and if so, how, would be a question for a tax specialist who is familiar with the tax laws in your country/state. In some US states, you can declare unpaid sales taxes when you submit your personal income tax declaration each year.