Ross Woolf's double strike '“ his second in five days '“ had given the Blues a 2-1 lead, but the afternoon's drama was about to unfold.
A goalkeeping clanger, an horrific injury and an extra-time winner ended Selsey's hopes of repeating last season's heroics in the competition.
The first half was dominated by the home side as they pushed Selsey down the Krooner Park slope time and again.
A shambolic defensive episode involving McLelland, Andy Brown and Billy Chatfield handed Gavin Muir a chance but his trickling effort rolled just wide.
Fine Camberley interplay set up Hinga Amara and only a smart stop by Rob McLelland denied him.
In a rare moment of respite for the Blues backline, Woolf's superb touch took him clear and Tom Ridley drove inches wide of Justin Gray's far post.
Green and Amara, Camberley's talented wingers, tormented the Selsey defence, for whom half-time came as a welcome relief.
After Woolf had poked a shot wide of the near post, controversy arrived.
Ridley's cross from the right dropped by keeper Gray and Joe Miles pouncing to tap into an empty net.
The celebrations were cut short by the assistant's flag, though, Miles adjudged to have fouled Gray, which enraged Selsey players and supporters.
Justice appeared to be done when Ridley's long diagonal ball was brilliantly thumped across Gray and into the far corner by Woolf.
Joy turned to despair when Greg Robinson handled in the box and John Swift smashed home the penalty.
Again Selsey took the lead as Woolf, firing across Gray in near-identical fashion.
Substitute Mark Pym had other ideas '“ first rolling home the simplest equaliser when McLelland dropped the ball at his feet and then sending a 20-yard half-volley into the corner, six minutes into extra-time.
Camberley's George Hardy departed in an ambulance after Rob Wimble's strong but fair challenge left him with a broken leg and dislocated ankle.
Selsey never got going after the 35-minute delay and Alun Morey was sent off for a second booking as the Combined Counties side played out time. "It was our own mistakes that cost us," said Hinshelwood.
"We gave away a stupid penalty and then an unfortunate error by Rob has ended up costing us the game."
SELSEY: McLelland, S Hinshelwood, A Brown, Miles, Chatfield (Nicholas 46), R Brown (Angell 112), Britton (Wimble 81), Morey, Robinson, Ridley, Woolf. Subs not used: Bines, Phillips.
* Selsey's cup exploits continued with a 2-1 victory at home to division-two Worthing in the John O'Hara Cup.
Selsey's superiority gradually began to show and they took a two-goal lead with fine headers from Dan Martin.
Worthing battled well but sometimes too vigorously '“ they picked up four yellow cards and one red.
Mark Lee needlessly gave away a penalty and Rob McClelland was sent the wrong way, but Selsey held on for a deserved win.