There is a hidden feature that allows Pit to haggle with Shopkeepers to lower the prices of items.

In the NES release, this is done by simultaneously pressing A and B on a second controller. In the Famicom release, haggling is instead done using a second controller with a microphone on it, by holding A on the second controller and speaking into the microphone to haggle. In the 3D Classics port for the 3DS, haggling can be done by simultaneously pressing the A button and either Start or Select.

The main catch to haggling is that the prices will only be successfully lowered if Pit's health is one point higher than the first number of the stage he is currently haggling on (i.e. Pit's health must be at 2 arrows when haggling in World 1 to successfully haggle). Otherwise, the Shopkeeper will raise the prices instead.

However if you fail to haggle, the prices will not change until the Shopkeeper's text finishes scrolling, creating a small window of time to buy something at its original price just before they increase.
Contributed by MehDeletingLater