Ok I think I figured it out (thanks @brickvoyeur for the test in PM). This is from an older article (2014) but probably explains it well:
2. State rules regarding shipping taxability are difficult to track.
For sales within one state, most states charge sales tax on shipping and handling, as long as those costs are included in the order.
The following states usually consider charging shipping that is part of an order a taxable activity, regardless of whether sales tax is part of the price or not:
Arkansas, Connecticut, District of Columbia, Florida, Georgia, Hawaii, Indiana, Kansas, Kentucky, Michigan, Minnesota, Mississippi, Nebraska, New Jersey, New Mexico, New York, North Carolina, North Dakota, Ohio, Pennsylvania, South Carolina, South Dakota, Tennessee, Texas, Vermont, Washington, West Virginia, Wyoming.
Conversely, the following states say shipping charges are not taxable if included in the price:
Alabama, Arizona, California, Colorado, Idaho, Illinois, Iowa, Louisiana, Maine Maryland (noted exceptions), Massachusetts, Missouri, Nevada, Oklahoma, Rhode Island, Utah, Virginia, Wisconsin.