People will be charged up to £3.50 to get their post when Royal Mail bans the use of traditional stamps.
The postal service is introducing barcoded stamps next January and says it will not deliver letters and parcels bearing the old ones.
They will be left at sorting offices with a “surcharge” – currently £2 a letter or £3.50 for a small parcel – to retrieve them.
Campaigner Malcolm Booth said it would “make life most difficult for the elderly and vulnerable – the very people who use stamps the most”.
He added: “Stopping people using normal stamps and making them pay to receive post will cause disruption.”
People sent items with the old stamps will receive a “fee to pay” card and be told to collect them from a sorting office.
But older people and those in rural areas – often miles from a mail depot – face long journeys to get post.
Royal Mail – which is battling week-long delivery delays in some areas – says Brits will be able to swap traditional stamps for the same value of new barcoded ones from March 31, via Freepost.
But campaigners warn some people will miss the announcement and only find out about the ban when it comes in.
Mr Booth added: “Many people have a stash of stamps at home. How many will not get the message about the changeover?”
Experts are calling for a switch-over period of several years where people can use both kinds of stamp.
Royal Mail said: “Non-barcoded stamps will remain usable until January 31, 2023.
“Either use up non-barcoded stamps you have or swap them using our scheme.
“Details of the swap scheme will be announced in due course.”