The new special edition "stamp cancellation mark" is being introduced to coincide with Valentine's Day.
Visitors to the website will find more information about the partnership and be encouraged to make an online donation to Help the Hospices.
On Welsh letters, the stamp cancellation mark will appear in both English and Welsh.
Denyse Busby–Earle, Director of Engagement and Inclusion at Royal Mail Group, said: "Our partnership with Help the Hospices, which is our first official charity partnership, has been extremely successful to date and we're very proud of it...
"We feel that now is a very good time to raise awareness another notch – not least because during February many people will be examining their cancellation marks closely to get a clue as to who might have sent them a Valentine’s card!"
To date, the partnership has generated over £700,000 for Help the Hospices and individual local hospices around the UK.
For more info click here: www.hth.org.uk/royalmail