give me yr opinion on ethical problem

[Cloud]

So, on Mar. 6 I ordered a book from Amazon Marketplace (you know, the individual sellers through Amazon, not Amazon itself). By the end of March, it still hadn't come, so I emailed the seller, who responded, Oh, I sent it out 2 weeks ago. Hmm. After another week or two, the expected delivery date expired, I emailed the seller--no response. So I asked, and received, a refund from Amazon.

We're talking a whole $5.88 here, folks.

After I received the refund, I got the book. Postmarked 2 days AFTER the refund was processed.

So do I refund the refund? Send the book back? Am I obligated to, since the Seller actually mailed the book after he paid me back my money?