PayPal customers have been urged to check their accounts now.
The warning comes after the firm brought in a new policy which will deduct funds automatically unless the system is being used enough.
Posting to his 1 million followers on Twitter, the 48-year-old said: "PayPal users warning.
"It is going to introduce a £12 inactivity fee. Here's how it works and how to avoid it. Feel free to share."
The MSE expert shared a link to his website which shows how PayPal customers can avoid the charge.
Customers will be charged a £12 fee from December 16, 2020, if their accounts have been inactive for at least 12 months in a row.
By "inactive", the firm means accounts where the owner hasn't logged in or sent, received or withdrawn money.
If there's less than £12 in your account, PayPal will zero it.
However, if you don't have money in your account, or your balance is negative, PayPal will not charge you.
The firm says it will warn inactive users about the fee 60 days, 30 days and then a week before they are charged.
To avoid the charge, all you need to do is log into your PayPal account or make a transaction on or before December 15, 2020.
If you're not using your account anymore, you can close it by going into "Settings" and selecting the "Account" menu.
Alternatively, you can transfer any money you have there to another account so the balance reads as £0.
Previously, Martin spoke about the self-isolation grant and said 4 million Brits are eligible for £500 payments from the government.
The grant was set up for those who can't work as they've been told to stay home by Test and Trace.
It was introduced back in September and can be claimed multiple times where necessary.
For more information, contact your local council through the government's website.