How do I upgrade Tom Nook's store in Animal Crossing for the Gamecube?

In Animal Crossing for Nintendo GameCube, Animal Crossing: Wild World for Nintendo DS, and Animal Crossing: City Folk for Wii, you upgrade Tom Nook's store by buying and selling things. Every Bell that a player spends or earns in the store increases a counter. (The counter is shared among all human villagers; NPC villagers do not affect it.) Once this counter passes a certain amount, the store will close the next day for an upgrade. On the GameCube and DS versions (not the Wii version), the final upgrade (Nookington's) requires someone from another town to buy something from your Nookway store after reaching the threshold, so you may need to buy another GameCube memory card.

The quickest legit ways to increase this counter are:

• Orchard Shake one-third of the fruit trees in your town each day (face a tree and press A) and sell the fruit. If an acre has fewer than 13 trees, bury fruit to make more fruit trees. Fruit species other than the one your town came with are more valuable; owning another memory card helps.
• Fishing Equip a fishing rod, throw the lure in front of a fish with A, and when it's pulled all the way under, press A to reel it in. More valuable fish come out on rainy days.
• Selling fossils Talk to Blathers to get Museum in Farway added to your contacts. Then grab paper from the dump, dig up unidentified fossils by using the shovel on asterisk marks, attach them to letters to Museum, and mail up to three per day. Once you get them back, you can donate the fossils to Blathers and sell anything he already has to Tom Nook.

Paying back your loan does not by itself increase this counter. But if you're paying back your loan, you probably sold things to Tom Nook to get the money, and that increased the counter.

To upgrade your Nook's Cranny to a Nook 'n' Go, you need to have bought or sold at least 25,000 Bells worth of stock and have played the game for at least 8 days. Additional information about it (time it is open and what it has) can be found here.