You can add sales tax when creating the booking proposal or an individual order. It's the very last setting. Just click the "Configure Taxes" link to get started.
On the next page, click the "Add A Tax" link.
You can then label your tax (this label will be visible to the client on the invoice), specify a tax percentage or fixed dollar amount, and the tax amount itself. You can also check the "Apply Tax on Shipping Costs" box if necessary.
Click the "Add Another Tax" link if you need to have multiple tax rates applied to an order (i.e. local and state taxes). You can also remove a tax by clicking the 'X' icon.
On the far right you can calculate the tax 1) against the final subtotal of the order or 2) against the retail price of the product and services within the order. The second option is useful, for example, if you are required to collect tax on just your physical products within your package and not your shooting time. This is why it's important to designate your products and services appropriately when creating them under Settings > Pricing.
Finally, under Tax Settings on the far right, you can determine if the tax should be applied 1) As a lump sum on the final payment in the order or 2) proportionally across each payment. If necessary, you can check the "Save As Default Settings" box if you want the system to remember your tax settings for the shoot or order you book. Hit the "Save" button and you're all set.