Yes. The bag tax must be applied to each disposable bag (plastic, paper, or other material including but not limited to compostable material) used to deliver goods or provided at self-checkout. It is recommended that stores place notification of the bag tax at each self-checkout location. Printable materials for signage are available on this webpage.
Online ordering services are able to charge a card an estimated amount when the order is placed, then change this amount after the fact depending on how many bags are used.