Humberside Police today appeal for anyone with information to come forward as it revealed at least two people had fallen foul of the scam.
A spokesman said there had been reports of two separate incidents at houses in Beverley and Willerby.
He said: "A man dressed as a postal worker said he had a parcel for the occupant but there was an outstanding postage charge that had to be paid before he could hand it over.
"The suspect claimed he could not take the outstanding £0.55 in cash and it would have to be paid by debit card.
"He then claimed the transaction had not gone through and moved away to get a better signal on the card machine – but instead left, taking the card with him."
In both cases, the card was later used to withdraw hundreds of pounds at nearby cash machines.
A 71-year-old man was targeted in the latest incident, which happened in Wheatcroft, Willerby, at around 9.20am on Thursday.
The bogus caller was described as being white, around 6ft and of heavy build, with short, brown hair and brown glasses.
He also had a beard, a double chin and wore a red T-shirt with a Post Office logo on the front.
The force also issued an earlier appeal after a 76-year-old woman in Lairgate, Beverley, had her card stolen in similar circumstances on July 25.
If you can help, please call 101 quoting incident numbers 16/78364/18 (Beverley) and 16/84320/18 (Willerby).
What should you do if an unexpected caller comes to your home?
Always ask for identification when you’re approached by someone on your doorstep. If you’re not sure, call the firm they claim to be from. If they’re genuine, they won’t mind.
Don’t assume that because they’re in uniform they are who they claim to be.
Don’t hand over any cash or your card or personal details.
Never allow someone into your home before you’re confident that they are who they say they are.