Shouldn’t “Or for any other reason” be part of another sentence and shouldn’t “Without accreditation…” also be part of the same sentence as something else?
eg. “You can’t do x…without prior approval from …”.
But I agree it’s probably best to contact CS before adding something like that to a delivery.
Also any conditions on the use of the delivery should be described in the gig description so they know before ordering.
From the terms of service:
Unless clearly stated otherwise on the Seller’s Gig page/description, when the work is delivered, and subject to payment, the Buyer is granted all intellectual property rights, including but not limited to, copyrights for the work delivered from the Seller, and the Seller waives any and all moral rights therein…
eg. you shouldn’t add extra conditions on use of the product only in the delivery message - any restrictions should be described in the gig description or somewhere else on the gig page (eg. FAQ?).
Also, putting it only in the delivery message (which you shouldn’t, since it’s probably against the TOS) could mean someone could order a voice over which was describing their gig (which might be a whiteboard video or anything that isn’t a voice over service) but then find out on delivery that they can’t actually use it because your delivery message says “can not be used on any other Fiverr gigs to promote or demonstrate your services”.