The Royal Mail is planning to charge customers for delivering letters and parcels according to their size rather than their weight from April 2006.
Large lightweight items can be more expensive to sort
Under the proposals heavier items are likely to become cheaper to post but larger ones will become more expensive.
Royal Mail said this would make pricing fairer as it was the size and shape of letters and packages that determined collection and sorting costs.
Industry regulator Postcomm will have to approve the Royal Mail's proposals.
The Royal Mail said the changes would end an "unfair" situation where people sending small weighty items, such as books, pay more than customers sending large lightweight items such as posters.
"Basically, this is about getting our prices to reflect our costs. Heavy but compact post will be cheaper to send," a Royal Mail spokeswoman said.
The Royal Mail told BBC News that it thought 80% of residential mail would not be affected by the changes.
As for the remaining 20% of mail, the Royal Mail said that roughly half would go up in price but the other half would be cheaper.
"We have had to go through the costs with the regulator. As far as our profits are concerned, it is a revenue-neutral move."
The regulator is expected to announce whether the Royal Mail can introduce the new charging regime in the next few weeks.
Similar postal-charging regimes operate in Australia, Japan and the US, according to the Royal Mail.
The UK's postal service market will be fully liberalised from 1 January 2006, with the Royal Mail's 350-year letter delivery monopoly coming to an end.
From that date, any licensed operator will be able to deliver mail to business and residential customers.
Royal Mail will still be required to provide a universal postal service for first and second class mail of one delivery and one collection each working day at a uniform price throughout the UK.