Can I control if Avalara/AvaTax charges tax on shipping and handling fees?

There are no settings on Pressero that can be changed that determine if tax rates returned from AvaTax include the shipping and handling charges in the taxable amount for an order. That is determined by the configuration on the AvaTax account that is returning the rates for your Pressero orders. There is a guide posted here, 7.1 - Taxability of Shipping Charges, but otherwise, any questions should be submitted to support for your Avalara/AvaTax account.