The card packs will include three cards: one that says "You Are So Amazing", one that says "What You Do Matters" and another which says "Not All Heroes Wear Capes".
In order to receive the pack, people need to fill in a form on the Hallmark website.
Royal Mail will be collecting, posting and delivering the packs of cards directly to the customer that they have been allocated to.
The customer will then need to attach a stamp or use the click and drop service to pay for postage in the normal way.
Amanda Del Prete, managing director at Hallmark UK, said: "We're inspired every day by the many heroes who are stepping up right now.
"These heroes are doctors, nurses, care workers, postal service workers, teachers, supermarket workers, cleaners, caretakers, delivery/truck drivers and many more doing the jobs that are right now keeping the world going.
"Each one of them deserves a huge thank you for everything they're doing in these unprecedented times.
"We hope our half million card giveaway will allow people to show the gratitude we know many are feeling right now - and to ensure those receiving the cards know just how much they're appreciated."