Full Slate does not offer a way to charge sales tax when clients pay online. However, if you're using Paypal Checkout Express (this is the standard Paypal offering), then you can set up sales tax in Paypal as follows:
You can charge sales tax when they use the PayPal Checkout Express option under Setup > Client Payments. Set this up in your PayPal settings (i.e. not in Full Slate). This setting is under My Account > Profile > Sales Tax in your PayPal account. Once it's set up, sales tax will automatically be added to charges coming in from Full Slate. Note, however, that the sales tax does not show on the first page clients see after leaving Full Slate and entering the PayPal pages; but it does show on the invoice summary after they enter their PayPal or credit card information.