Of course, we're assuming the receipt has pertinent info, and it's highly possible that it doesn't.
Let me explain my thought process to you:
As a parent, Cindy is not obligated to turn over evidence that could be used against her child without a warrent for that evidence. It takes five minutes to get a judge to sign a warrent. LE gets the receipt, Cindy gets to sleep at night knowing that she did not put a nail in Casey's coffin, she also preserves her relationship with Casey on the off chance that Casey ever decides to 'fess up. And all it did was take a few minutes longer!
I'm a mom. I can see myself telling LE, could you go get a search warrent for this? ESPECIALLY in a state with the death warrent.
Caylee is gone. Cindy probably knows deep in her heart that Caylee isn't coming home, ever again. Cindy may hate what Casey has done, but I don't think Cindy will ever hate CASEY. Love for a child is probably the most powerful force on earth. I just tried to put myself in Cindy's shoes, and the above is what I came up with.