Changes to the cost of posting letters and packets come into force today, based on the shape and thickness of an item as well as weight.
The Royal Mail said postage for more than 80 per cent of items, including business post, will stay the same or be cheaper.
The change was being introduced so prices reflected more fairly the actual cost of collecting, sorting and delivering mail, said the postal group.
Royal Mail director of pricing Lorna Clarkson said: "We want to make it as simple as possible for our customers to understand. That's why we've introduced the three size categories.
"There are ways that people can ensure that they do not pay any more than they have to."
Consumer group Postwatch said it was concerned about how the new pricing structure was being introduced.
A study of more than 300 post offices in the first week of August by the watchdog showed that a third did not have any information for customers on the so-called Pricing in Proportion (PiP).
A Royal Mail spokesman said the survey results were not surprising as the Postwatch research was conducted before the distribution of templates, posters and leaflets to Post Office branches had been completed.