Poor service

I had bough a service kit and it included an aircon filter, however my mechanic didn't use it. So, i took it back still sealed and i was told that i should have returned this the next monday, however it is not stated on the return policy on the receipt. I am so disappointed, what a poor service. I was told that i should give it to them and they they will only reimburse me my money once it has been bought by another customer.