The postal watchdog wants to stop unfair surcharges being imposed on stamped letters and packets sent by customers from their Post Offices.
Consumer Focus has told the BBC that the guidelines on what to charge for parcels and packets are not always being strictly followed.
The problem applies where mail is not weighed or stamped correctly.
Royal Mail staff check items at random to see if they have been correctly chargedRecipients then have to pay the shortfall and a £1 fine at their local sorting office.
The issue came to light thanks to Maggy Edwards, who frequently mails out newsletters from her home in Coventry.
She is a big fan of the Star Trek actor Leonard Nimoy, and helps run his official fan club. But she is certainly not a Post Office fan at the moment.
That is because after taking around 100 identical newsletters to a local sub-post office for weighing, sizing and posting, more than 20 of her club members were each surcharged and fined £1 for underpayment at their own sorting offices.
"Some of the members blamed me, perhaps thinking I just bunged a few stamps on and put the newsletters in the nearest letter box," Maggy said.
"But it was the local counter clerk who weighed and measured the items and told me what stamps to put on," she said.
Ms Edwards was so upset at what happened that she wrote to each affected member personally, reimbursing them out of her own pocket.
Maggy Edwards fears having to pay more excess charges for her members
Now she says she is nervous of posting batches of fan club mail.
She says the news letters all fitted the measuring frame in the post office, and has no reason to think the counter staff gave her the wrong information.
She then took the issue up with the Royal Mail, the Post Office and her local MP, but says she got nowhere.
Her favourite Star Trek character Mr Spock would doubtless label what happened "illogical".
So how can it be that Post Office counters and Royal Mail sorting offices seem to be living in parallel universes, where different charges apply?
The post and packet pricing structure is quite complicated”
David Stubbs
Postal expert
It appears that, no matter what the counter staff may decide, the sorting officers are the final arbiters of whether you have paid the correct price.
The surcharges and "administration fee" are imposed by revenue protection staff who pull out items at random to see if the correct postage has been paid.
David Stubbs, an independent postal specialist working in the industry, says the problem partly arises because of a lack of complete co-ordination between counter staff management and the Royal Mail itself.
"A lot of it is down to a lack of training in the post offices around the tariff structure - and also because the post and packet pricing structure is quite complicated.
"There are five different prices for letters, another 10 for packets and then you go into parcels," he pointed out.
The Post Office said it was sorry for any difficulties experienced Mrs Edwards, but had no new information to add.
Nigel Woods of Consumer Focus will ask the Royal Mail to change its policy
"We aim to take the greatest care in advising customers on postage costs, and the overwhelming majority of all mail has the correct postage paid," it said.
It points out that if it did not take a strong line on wrongly stamped items, many more people would put too few stamps on items, or maybe none at all.
Nigel Woods of Consumer Focus said he did not agree that training was much at fault, but said that it was wrong that the price quoted at the counter could be changed later and the recipient surcharged.
So his organisation will talk to the management of Royal Mail and the Post Offices to get things changed.
"We'd like to see if there can be a system in place so when a customer buys a stamp over the counter and the mail gets sent from the post office through the system, when it it is pulled out by revenue protection people, they can see it's been purchased over the counter and will let the mail carry on," said Mr Woods.
"They can then take it up with Post Offices Ltd internally," he suggested.
Maggy Edwards of the Leonard Nimoy fan club says she is now nervous of posting her newsletters. Mr Spock himself has made no comment.
With 70 million items boldly going through the system daily, measuring and weighing post consistently may not exactly be the final frontier, but the problem certainly needs a space-age solution.
and the same too for the 100g and 500g Special Delivery stamps due out soon ? 