"Souvenir Covers of the World" collections produced by collectible companies receive some popularity among elementary collectors, popular themes include ships, pets, WWII and royal events. While some of these covers have first day of issue cancellations or proper postal cancellations, some do not, collectors should be well alert of this.
British collectible company Bentham gives an illustrative case, this cover was produced for their "cats" series:
The cover features an oversize cachet on the left half, and a 1999 Cambodian postage stamp (Sc#1822) tied by a "1999 Cambodia Cats" "cancellation" on the top right corner. The "cancellation" is not a postal cancellation of any kind, in fact it is a private made cachet, or a rubber stamp impression some people might call it, it is not even an official cachet from the Cambodia Post. All 6 stamps of the set (Sc#1818-1823) have their corresponding souvenir covers by Bentham.
For comparison, the following is a cover from the same series, franked with an Antigua & Barbuda stamp (Sc#2014). Notice the cancellation is a valid postmark.