MP Sarah Atherton sent voters campaign leaflets with the wrong postage - meaning they had to pick up the cost themselves.
One constituent warned people to "be aware" after she forked out £2.50 for the political promotion.
Writing on Facebook the woman said: "People shouldn't be charged for this."
Ms Atherton's office apologised, saying a "handful" of people had been affected and that they would be reimbursed.
Posting on Facebook group Wrexham Town Matters, the woman said she wanted to warn people before they "waste their time and part with their cash".
She said she initially received a letter from Royal Mail saying it had a letter that required payment.
"It turns out the mysterious letter was actually from Sarah Atherton MP," she said.
"I've wasted time and effort, and not forgetting paying £2.50 either, on her campaign advertising all because she used invalid stamps.
"It's not on, people shouldn't be charged for this."
She only paid as she didn't know what was inside.
"Be aware everyone," she said.
Royal Mail said it had a number of the missives at its Wrexham sorting office.
The Tory MP's office said: "We have been made aware that this has happened to a handful of people and apologise for the inconvenience caused.
"We are looking into what went wrong so it does not happen again."
The spokesman said anyone affected would be refunded if they got in touch.
Ms Atherton said people wanted to make the story "political" but she wanted to concentrate on the healthcare message in her leaflet.
Royal Mail said surcharges were applied when packages were posted with insufficient postage or invalid stamps.
Last year Ms Atherton hit the headlines after Wrexham AFC fans made fun of her for calling the club Wrexham Athletic.
And in 2020 the Welsh Conservatives apologised after she appeared in a party political broadcast filmed at the Houses of Parliament without permission.