Rowf and Rhuff
From the Super Mario Wiki
Rowf and Rhuff are a Doogan father and son who run Toad Town's only badge shop in Paper Mario. They constantly supply Badges for Mario's shopping needs, though at one point Mario must retrieve Rowf's Calculator from the Shy Guy's Toy Box. Their shop opens after the end of Chapter One. Although expensive, the player should buy as many as possible, as some of the badges are one of a kind and very helpful.
A Toad from the docks in Toad Town mentioned that Rowf and Rhuff used to sell rare foods. The Toad also mentioned that he set up a shop in a different town a long time ago while selling rare foods before moving to Toad Town to sell badges. The Toad lastly mentioned that Rowf's wife used to work with him a long time ago in the business but ended up with no profit. The Toad muses what happened in that time.
At the end of the game, they leave to sell at another place.
Mario will eventually get a letter from the pair stating that Mario is a great customer and that he should come back to their store and also that their badge prices are 0% off.
If Mario buys all the badges they have by the end of Chapter 5 and visits them again, Rowf will tell Mario he'd bought him dry and will thank him for being such a frequent customer. At the end of the game, if Mario stops by the closed badge shop on the way to Princess Peach's Castle, Rowf says he plans on taking his profits (a total of 1,650 coins if you purchased every badge) and moving on to another town. He claims he's only staying for the moment because Rhuff is excited about the festivities going on.
Badges on sale
In addition, Rowf awards Mario with the I Spy badge for returning his Calculator.