Yes but the example here would be someone in Cali ordering a jerry can from Idaho and the company in Idaho mailing it to Cali. Travelling to Idaho to buy a jerry can and bring it back would be fine, but in this case getting it to Cali is a part of the purchase. The purchase is being made across state lines for something that is legal in the state the transaction is made (Idaho) but illegal in the state it’s sent to (Cali). It basically boils down to whether the act of mailing is illegal, I think.
I’m sure there must be an established precedent or something already.