Are there any expressions I should not use in the story if the reward is a pet diffuser?

Pet 'diffuser' falls under the category of 'air freshener,' so you cannot use expressions related to 'odor suppression,' 'calming effects,' and similar claims. 

Consumer chemical products cannot describe efficacy outside their registered intended use/product category, regardless of whether test reports exist. 
Therefore, even if you have an odor elimination test report, you cannot use 'odor suppression' expressions for pet diffusers. 

Additionally, claims that the fragrance of the reward provides calming effects to pets is an expression that cannot be used, as it is difficult to objectively substantiate and falls outside the scope of the diffuser's registered efficacy.