Customers might have to pay surcharges this year for some Christmas cards that arrive without sufficient postage, though the fees were waived last year.
Consumer watchdog Postwatch warned that an amnesty for cards without enough postage will not be repeated this year.
New sizing and pricing rules began in August 2006, but were not enforced over last year's festive season.
In the spirit of the holiday, customers should get the same deal this year, said Andy Frewin, Postwatch's director.
"What we are hoping very much is that Royal Mail will apply the spirit of Christmas rather than the letter of their rules and only ask for the surcharges when it is blatantly under-stamped," he told BBC's Breakfast.
Mr Frewin said customers who receive a card with insufficient postage will be given a notice to that effect and will then have to go to the post office, queue for service and pay £1 plus the missing postage before they can collect their cards.
A spokesman for Royal Mail told the BBC that while there is no official deal to ignore the surcharges over the coming weeks, they will be applied using "common sense" over the holiday season.
In bringing in the changes last year the Royal Mail said 80% of post should cost the same amount to send.
Royal Mail said the charges reflect the cost of sorting large letters, which has to be done manually instead of by machine.
Postwatch has encouraged consumers to familiarise themselves with the revised pricing and sizing in order to avoid the extra fees and inconvenience.
THE PRICE OF POSTAGE
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