Parcelforce will today become the first national parcels carrier in Britain to deliver on Sundays.
The express parcels business of Royal Mail said the new service will particularly benefit online shoppers.
The first retailer to sign up is Fortnum and Mason.
The Communication Workers Union said it had reached an agreement that Sunday working would be voluntary, while negotiations will be held on new shift patterns.
National officer Terry Pullinger said: " Talks with Parcelforce management established a very important principle as far as employment security is concerned and we are delighted that we have had an assurance that permanent employees will be used to deliver this new exciting innovation to customers."