Our current cat looked pretty good but had some major hidden health issues we learned about in the following weeks. The shelter did offer an exchange or refund although it wasn't real gracious, but we didn't have the heart to toss him back after he had settled in so quickly (he "owned the place" immediately after leaving the cardboard carrier... it was like he had been born and raised here).
We dropped a big chunk of money on him for some major surgery that was mostly successful, but he will always require attention. He's family now, for better or worse.
They are family aren’t they? Offering an exchange or refund is for inanimate objects IMO, I would have (and have) done the same thing you did. And yes it can get expensive, shopping around for vets in our area we were able to find a good and reasonable one. Good luck with yours.
Almost 4 years ago I adopted a kitty from a refuge, already 7 years old. Severe allergies, yeast infection in the ears, digestive problems...3 weeks ago I had to end her life as her body was full of cancer. Best kitty I ever had, cost a lot of money but don’t regret it. I gave her all my love and she clearly adored me. They leave a hole in your heart.