We went to a pet hospital because my dog was scratching her ears and they looked infected. I got a pill for my dog to take every day. And it worked.
So we continued to give her the pills until gone. I then returned for more pills.
They were out so we got a creme instead. But we had paid for the pill first and that was more expensive than the creme we got so they told me that I would get the money back the next time I come. And I went to that hospital again and wanted to ask for the money back.
But as you can see above my English is very childish and informal.
How would you make such a request clearly, simply and formally?