A common adventure

I wouldn't leave it up to chance whether they embark on the adventure. State at the beginning of the session who the PCs and their employer are, the opportunity that has come before them, and that they've already decided to go after it. That's the premise of the one-shot. If they walk away from it, there isn't an adventure. Their individual personalities and how they go after the treasure is up to them, of course.

As for encounters, keep it combat light and don't use too many combatants at once when you do. Just 2 goblins is a very deadly encounter against a group of commoners using the standard NPC stat blocks, but I'm not sure how literal you were when describing them as commoners. If you do use the aforementioned stat blocks, they'll usually get KO'd in one hit, and they'll get hit roughly 70% of the time (AC 10 and 4 HP vs +4 attack bonus for 1dX+2 damage). One way you can tweak combat in their favor is to give them all ranged weapons and make all the NPC enemies melee combatants. If they don't have ranged weapons as part of character creation, present the opportunity to steal/borrow some crossbows from the town guard/militia or something.

Their strongest weapon should be planning and cunning use of the environment, however. Some ideas for meaningful decisions/challenges:

1) A formidable enemy (no common goblin!) stands at the edge of a chasm with its back turned to the PCs when they enter the area. They can easily sneak past him (auto success), but then they might have to deal with him on the way back. Alternately, they can try to push him over the edge. If you can plan for methods for pushing him over the edge other than an actual push attack, that'd be great. Telegraph those possibilities where appropriate.
2) An area filled with toxic gas. At the far end and around a corner, they hear singing/chanting/growling/whatever. They can run through the gas, holding their breath to avoid being affected, but the noise will likely alert whatever's at the far end. If they try to move quietly through it, they must succeed on a Constitution save or suffer the poisoned condition for the remainder of the adventure. Or maybe they can find some way of clearing the gas out. Perhaps it's flammable (but the ignition will alert the far end), or maybe it can be neutralized by breathing through a pee-soaked rag. How the PCs might arrive at that last idea I'll leave to you!
3) Gobbos love them some traps. No need to get especially creative here. Trip wires with falling rocks, pit traps, etc. One near the entrance and one near the treasure would be appropriate. The one near the entrance could also include a goblin lookout who, if he detects the PCs, will try to lure them into it. I would be obvious rather obvious about telegraphing his intent, though.

The "typical adventuring day" for standard PCs is theoretically 6-8 encounters. For level 1 PCs, 3-4 (or even fewer) is more appropriate IMO. For commoners? 3 should be enough to strain their sense of courage. Hopefully the 3 suggestions above work for you. If not, or if you simply want to add more, a standard combat encounter (or two) should be fine. Again, though, be very careful about using more than 1 or 2 goblins at a time. You can include more if there's an extreme tactical advantage for the PCs (they're firing from elevation and the goblins won't reach them for 2 or 3 rounds, for example).

I'd also be generous with letting them rest to recover. Short rest = full HP. They only have 4 HP on average. So they bandage their wounds, grit their teeth, suck it up, and press on. When all is said and done, maybe they'll be level 1.