Royal Mail is introducing a new pricing structure that will see the cost of sending mail based on the size, weight and thickness of items.
Sorting larger letters is more expensive, says Royal Mail
To spread awareness of the new prices, which start on 21 August, it is sending a leaflet to every UK mailing address and launching a £10m ad campaign.
Royal Mail says 85% of stamped mail will stay the same price or be cheaper.
It says two-thirds of business mail will be cheaper, but some firms fear a big rise in their mailing costs.
The new charging scheme, which Royal Mail is calling "Pricing in Proportion", will see three size formats:
- Letter - e.g. standard birthday card, with a maximum 5mm thickness. Cost: 32p.
- Large Letter - e.g. the size of a magazine, with a maximum thickness of 25mm. Cost: 44p.
- Packet - anything larger than a large letter. Cost: Between £1 and £4.74.
Royal Mail says it has introduced the charges to reflect the cost of sorting large letters, which has to be done manually instead of by machine.
Caroline Elsie, the managing director of the CMD Personnel Consultancy, told the BBC her company would end up paying more because it posts a lot of material in large envelopes.
"We are going to see an increase going up from 32p to 44p for a lot of our mail.
"This will probably cost us an extra £120-130 a year - just another burden for a small business."
Ms Elsie said she would have to pass the cost on to her customers.
The Greeting Card Association has issued guidance to its members and has come up with a logo that they can use to show customers how much it will cost to post a particular card.
The association has suggested its members re-design their products to comply with the new sizes and said some companies were already looking at slimmer age badges for cards.
Royal Mail is providing all households with a paper template that demonstrates the new pricing structure, with businesses receiving a cardboard version.
Post Offices will have a plastic template that customers can use to test letters, and some stationers will sell a plastic ruler that has a 5mm slot for testing thickness.
Consumer watchdog Postwatch has said it will watch to see what impact the price changes have on customers who are unaware of the changes and the possible knock-on effect for Post Office queues.
CHANGES TO ROYAL MAIL CHARGES
Letter: Maximum size - 240mm long, 165mm wide, 5mm deep; maximum weight - 100g; cost - 23p (2nd class) or 32p (1st class)
Large letter: Maximum size - 353mm long, 250mm wide, 25mm deep; maximum weight - 750g; cost - 44p to £1.31
Parcel: Anything over 353mm long, 250mm wide, 25mm deep, or 750g in weight; cost - 84p to £4.74 (+85p per 250g above 1.25kg)