Following a hard-fought 0-0 draw at the Impact Arena, courtesy of a brilliant defensive effort, United prevailed on spot-kicks through Michael Bostwick, Zak Mills, Jacob Hazel, Deji Sotona and Michael Gyasi.
Cameron Gregory saved to deny Harry Perritt, with Gyasi subsequently scoring the winning spot-kick to spark jubilant scenes in the packed away end.
United, unchanged once again, stood up to the physical challenge posed by an Alfreton side one place and five points higher in the final standings.
Chances were at a premium in a tight first half, with the clearest chance falling to Kelsey Mooney eight minutes before the break - with his rising drive narrowly clearing the crossbar.
Jai Rowe produced a brave piece of defending at the back post in the early stages of the second half, warding off the threat of ex-Pilgrim Jordan Thewlis.
Alfreton had a hat-trick of late chances, with substitute Gerry McDonagh heading narrowly wide, Gregory denying Billy Fewster and Thewlis lifting a gilt-edged chance over the bar in stoppage time.
Extra-time was another cagey affair - with the main talking point seeing Ian Culverhouse red-carded from the dugout for delaying an Adam Lund long throw-in.
McDonagh, Josh Clackstone, Lewis Salmon and Liam Waldock all scored in the shootout for the Reds - before Gregory's crucial save to deny Perritt.
Alfreton (4-2-3-1): George Willis; Josh Clackstone, Kennedy Digie, Dwayne Wiley, Nathan Newall; George Cantrill, Adam Lund; Billy Fewster (Harry Perritt 112), Liam Waldock, Jordan Thewlis (Lewis Salmon 90); Jake Day (Gerry McDonagh 80). Subs (not used): Shaun Brisley, Jake Askew (gk).
Boston (4-1-2-1-2): Cameron Gregory; Jai Rowe, Michael Bostwick, Zak Mills, Brad Nicholson; Martin Woods (Sam Smart 72); Jordan Richards, Dylan Hill; Keaton Ward (Deji Sotona 72); Kelsey Mooney (Jacob Hazel 83), Jimmy Knowles (Michael Gyasi 97). Sub (not used): Tom Leak.
Referee: Darius Bradley.
Attendance: 2,093 (929 visitors).
PIC: Dan Westwell.