Webroot and Symantec researchers are warning about two different spam runs.
The first one consists of emails pretending to be from "FedEx Online Billing" and supposedly contains a new invoice that has to be checked out and paid.
Unfortunately, the offered link to the login page for the account leads to websites hosting the Blackhole exploit kit, and ultimately to the users' computers being infected with a Zeus/Zbot Trojan variant.
The second one sends out "tracking details" and urges users to print the linked receipt in order to collect a parcel that was sent to them but that the courier was unable to deliver:
The link takes the users to compromised legitimate pages where the malicious PostalReceipt.zip file is made available for download.
The file is actually an executable of the same name, and contains the Smoaler Trojan, which downloads additional malicious files and may also open a back door on the compromised computer.
As always, users are advised to regularly update their OS, software and AV software in order to minimize the risk of infection, and to avoid clicking on links in unsolicited emails.