Coldstream were looking to reach their first cup final in eight years as an independent club, and they chould have been a few goals clear by half time with an enthusiastic opening 45 minutes.
A lack of composure, poor finishing and rash decision making in the final and defensive third cost them dearly.
This was highlighted in misses by Ledgerwood, McGregor and Simpson and a handball by the experienced Ford to give Biggar a penalty and a lead going into the interval.
The second half belonged to Biggar. They stifled any home attacks and controlled the game with their two central midfielders with good use of the ball.
Simps had a good chance to equalise from a Wolfie flick on and Waugh was clean through only to be ruled offside.
With two mins remaining and the Ammies pushing on, the inevitable happened. A corner was pumped in and with the Biggar sub struggling to reach a ball he stuck out a leg and it deflected off an Ammies shoulder and flew into the top corner.
Then, straight from the restart, Coldstream gave the ball away. Biggar knocked it into the box. Ref Campbell pointed to the spot after the Biggar striker fell over and the same striker coolly slotted it home to take his side into thefinal, which will be played this weekend against St Boswells atEarlston.