The half-price "Brazilian-style" lacy briefs were part of another pre-wedding shopping trip for thrifty Middleton, 29, who also bought two bikinis, ahead of her wedding to Prince William.
"I certainly didn't expect her to be browsing at the bottom end of the range," the Sun quoted one shopper as saying.
Middleton's red-faced Royal protection officers waited outside as she browsed for smalls at lingerie store Goldenpoint in London's King's Road.
She was also spotted returning to the street in West London's Chelsea, for more underwear.
She trawled the lingerie department of Sloane favourite Peter Jones and picked up two Calvin Klein bras for around 30 pounds each - one black and one nude.
In the department store she also bagged a pair of sunglasses for her trip even though they were the tester pair tried on by hundreds of people before her.
"Kate fell in love with the sunglasses but they were the last pair and had been sitting on the rack for ages. She said it didn't matter if people had worn them before, she liked them too much," a source said.
At High Street chain Whistles Kate had to swap a pair of 150 pounds blue-and-while spotty trousers in her usual size 10 - because she has dropped to a tiny size 8 in her pre-wedding weeks.
While paying for the trousers and two tops at the till, she told staff that they were for her