The eTM in the lower-left
The eTM (e-Card Transfer Machine) is a machine in the Post Office that can upload a temporary program to the e-Reader that can transfer the data from scanned e-Cards to the GameCube and send mail to the player. It was not present in Wild World and onwards.
Related[]
In Able Sisters, the player can scan design cards or upload the program. Since there aren't passwords on design cards, the player has to scan them in.
When making a Town Tune, the player can press the "R" Button to scan K.K. Town Tune cards or upload the program. Since there isn't a password on Town Tune cards, the player will have to scan them in.
The Message Board lets players enter in villager cards or upload the program. There are passwords on the villager cards that players without an e-Reader can give to Tom Nook.