Paypal is definately not safe. Anything used with a credit card leaves a trail to anyone with access to you credit records (or if you have your credit bills laying around). And credit/bank records are usually the first thing the police look at if they have probable cause to. Of course all it will say on the records is Paypal (which could be ebay auctions, something I think most internet users have taken part in at one time or another) and it would be their responsibility to follow up on, but it wouldn't be very hard- Paypal will cooperate with them. If it's a transaction for several hundred dollars, I'm sure they'd look into it.
WU isn't totally safe either. You don't want any WU receipts around the house. Plus suspected dealers get flagged and if you're sending any money to someone suspected of illegal activity a message appears on the screen at the place sending the money for you, and the cops could be notified.
On a side-note, when my house got raided something I thought was kinda weird was they didn't look through any of my unopened mail laying around the house (sometimes I get bank records or bills and don't open it for some time if at all). It struck me strange till I figured out mail is federally protected and they didn't have a warrant to open my mail.