Located in the village near the Burrow (presumably Ottery St. Catchpole), the paper shop is a Muggle shop that Fred and George enjoy visiting, because there’s a pretty Muggle girl who works there who enjoys Fred’s card tricks (HBP16).
I can't help but wonder if Fred was impressing this girl with genuine magic (which would have run afoul of the rules against underage magic and magic in the presence of a Muggle) or just practicing with the kind of marked cards they eventually sold in Weasley's Wizard Wheezes. My guess is that he was doing Muggle magic tricks. After all, the twins are adept at many Muggle skills, including picking locks and sneaking through dark houses in the middle of the night.
And tell me I'm not the only one who finds the image of Fred and George visiting the shops in a Muggle town to be a delightful one? Someone please write the fanfiction story!