Deniz Mehmet pulled off two penalty saves to earn United a 4-3 shoot-out victory at McDiarmid Park.
The bonus point saw United finish on eight points, and they currently sit top of Group C on goal difference ahead of Peterhead, who recorded a shoot-out win following a 1-1 draw against Kelty Hearts in Fife.
Micky Mellon's side will be knocked off the top come Saturday, when Saints face Peterhead in the battle for top spot, and could also fall out the top two.
Even if they remain second, they could miss out on being one of the four best runners-up who will join the second-round draw with two groups already beyond them.
Saints had the better of the chances on the night and were denied a clear first-half penalty when Mark Connolly swiped the legs of Callum Hendry after the striker had killed the ball. Referee Don Robertson waved play on.
Jeando Fuchs made his first United start as both teams made six changes and each side struggled to find their rhythm in the attacking third.
Paul McMullan looked lively for the visitors but could not find a finishing touch to some promising moves.
Hendry came close from long range for Saints and Mehmet saved from Scott Tanser after dropping a cross.
Saints stepped up a gear after the break but could not get the end product.
David Wotherspoon shot over first time from 14 yards before Chris Kane set up Hendry for a glorious chance but the striker blazed well over.
Mehmet made a decent stop from Shaun Rooney's header before proving the star man in the shoot-out.
Mehmet saved Stevie May's penalty before appearing to stop Craig Conway's effort only to see the ball spin back several yards and over the line.
Connolly blazed over for United but Mehmet immediately pulled off a stunning save from Tanser before Nicky Clark sealed the bonus point.