Hello there! This is my first time using this forum, so apologies for any lack of etiquette, general stupidity, etc, etc.

I built a kossel mini and am very close to being able to print, with everything calibrated that I can calibrate EXCEPT FOR THE EXTRUDER. For whatever reason it doesn't turn when I tell it to using Pronterface, even though the text box says that it has successfully extruded the set amount of filament. I have tested the motor on a different driver and it turns fine, and I even swapped the driver in the E spot for a spare, but to no avail. The voltage for the E driver is .438, and changing that didn't help either. I'm thinking it's some problem buried in code somewhere.

Short of replacing the MEGA board, I have found no solutions online that have worked or are applicable. I'm using Marlin for firmware.

PLEASE HELP, I don't know what else to try

The difference between your printer and the thing you used to test the motor is the cable from the controller board to the extruder motor in the printer. When you tested the motor you didn't use THAT cable. Steppers don't turn if there's an open connection, a shorted connection, or a couple swapped wires. Check the cable.