CAUTION regarding Letter - Unsolicited Material:
Please be aware that the vendor might not agree that your library has a right to retain material without paying for it. This is especially true in the case of "related material"shipped pursuant to an existing subscription. The vendor might argue that, at the time of the original order for the title, your library consented (in very small print, to be sure) to receive and pay for (or return at your own expense) all supplementary, replacement, and related material.
A dispute concerning unsolicited merchandise might not mean much if you have no on-going relationship with a vendor. For instance, if a publisher you have no intention of dealing with in the future, sends you something you have not requested or fails to heed your request to cancel a subscription, retaining the merchandise without paying for it will probably have no adverse consequences for your library. However, if you have other subscriptions with the same vendor that you wish to maintain, failing to either pay for, or return, an item without obtaining permission from the vendor, might result in your other subscriptions being suspended for non-payment of the disputed item.
In cases in which you wish to maintain a friendly relationship with a vendor, working the dispute out in a way both parties are comfortable with (perhaps, by your paying for return shipment of the item this time in exchange for a written promise from the vendor that such shipments will not be made in the future) is probably preferable to simply keeping the material without paying for it.