Royal Mail is introducing a new fee of £1.50 for a letter with insufficient postage, and £2 where nothing has been paid.
A flat fee of £3 will apply to a small parcel, with the new charges coming in on Monday.
Royal Mail said the new charges were simpler, adding that surcharge handling fees have not changed for a decade despite rising costs.
Under the current structure, the surcharge is £1, plus any remaining postage due.
The changes are not expected to increase the average amount a typical household pays in surcharges in a year, said Royal Mail.
Stephen Agar, managing director of consumer and network access at Royal Mail, said: "Surcharging customers is the last thing Royal Mail wants to do. We make it as easy as possible for senders to attach the appropriate postage to their letters. The vast majority of consumers get it right most of the time.
"On the occasions where they don't, the surcharge helps to cover the extra cost of handling items that have been underpaid or have no postage attached."
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