1. You clearly state your policy before selling her the chin.
2. YOU still upheld your 15 day health guarantee!
3. You were never aware of any health problems to begin with, and she never contacted you about any problems.
4. It has been 19 days, therefore your guarantee is now void, that is just how it is, business is business and any other store/breeder would uphold to that rule.
5. Even if it were within the 15 days, your policy states you need a necropsy and a vet note...well, she has neither after 19 days.
Case and point, do NOT give her another chinchilla! You did your part as a breeder and upheld your end of the bargain. This lady needs to learn her lesson.