Decide & Deliver claims to provide a quick and easy shopping service, allowing users to complete address forms with the stroke of a key and pick up shopping from one of 16,000 Post Office branches.
The campaign, through Proximity London, focuses on the service's aim to make shoppers feel they are shopping at a local retailer.
The copy in the emails reads: "Just send it to the usual address" and "Just send it round my Gran's".
New users registering for the service will be entered into a competition to win a year's supply of chocolate from www.thorntons.co.uk, one of Decide & Deliver's retail partners.
Decide & Deliver has signed up 5,000 internet users since it launched in April.
The service's website has been redesigned, also by Proximity, to prepare it for an increase in traffic from the new campaign.
Catherine Campbell, head of home shopping development for Royal Mail, said: "Anyone who shops online is always looking for greater convenience and can only benefit from the service.
"Our joint consumer promotion on the Royal Mail portal with thorntons.co.uk shows the value attached to the service for retailers who are already participating."
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