I believe they gave them the authority to regulate interstate commerce, as that power was understood and intended at the time it was granted.
Right, but the only insight you can give into how ONE of them understood it is from a letter that Madison wrote almost a half a century later. Do you actually think that the Founding Fathers ever envisioned a day when interstate commerce was the norm in nearly 100% of goods sold in this country? Do you think that they envisioned a day when a person on a computer could buy something from overseas with the click of a mouse without the state government being able to even know about it?