How long until Amazon notices I've kept their "lost in transit" package?

[remake]

Okay. So last month I ordered quite a few things from Amazon. I waited a few weeks, the package got lost in transit, so they offered me a refund.

But a few days later the package arrived. I opened it and already watched half the things in it.

I'm planning on paying it off, but I can't do so until next week. How long do you think Amazon finds out that the package was delivered and I have it? When I do plan on paying for it, what's the best way to explain to them? "Oh, hey, I got your package a couple weeks ago and I just now want to pay for it."?? Would I be too extreme if I asked for a discount since it took way too long to get to me?

To be exact, I ordered the package on May 15th and it didn't get here until June 6th. I requested the refund a few days before that.

Help please.