What is my responsibility here?

[[censored]]

A month ago I bought some tropical (live) fish on a site similar to ebay but for tropical fish. I got them last week completing the transaction.

Today when I woke up there was a package shipped to my address from the same guy. Apparently he meant to ship some fish to a dude named Dan but mistakenly used my address.

The cost of the fish was $45 and the shipping was $27. I don't like these type of fish (guppies) and would never buy them for my tanks.

The fish are still sitting in their unopened bags. If DHL were to return for the package they could be shipped. However, it is unlikely they would survive another 24 hours in the bags. If opened I do not have the capacity to repack them properly.

I sent the shipper an email notifying him of his mistake.

What is my responsibility here to the shipper and monetarily if I were to end up keeping them. If they are not picked back up by UPS by the end of the day and are facing death?