what is your expiration date of your products? Do these products ever go bad?

Technically speaking XS blends have a best if used within 3 years (36M), a good rule-of-thumb is that if stored in a cool/dark place, like a dresser or closet, they should hold up well for 3 or more years. We would wager that after 3 years you may begin to experience a drop in efficacy. If you keep your blends in your car console exposed to heat/cold, the efficacy drop would be much quicker. 

Because we use some perfumers alcohol in XS blends some evaporation might occur.