I suspect you and the author are using different definitions of "paid for".
To rephrase, in order for the tube of toothpaste to be on the store shelf, all the resources required to create it must have already been expended. The workers got their paychecks last week, the ingredients were already purchased. No further resources are required to put the tube on the shelf, and your payment cannot influence whether it gets there, because it's already there. Your payment can only influence whether there will be another tube of toothpaste to replace it later.
To rephrase, in order for the tube of toothpaste to be on the store shelf, all the resources required to create it must have already been expended. The workers got their paychecks last week, the ingredients were already purchased. No further resources are required to put the tube on the shelf, and your payment cannot influence whether it gets there, because it's already there. Your payment can only influence whether there will be another tube of toothpaste to replace it later.