In a small bowl, whisk together soy sauce, brown sugar, cornstarch, and red pepper flakes (if using). Set aside.
Heat vegetable oil in a large skillet or wok over medium-high heat.
Add the shrimp to the skillet and cook for 2-3 minutes until they turn pink and opaque. Remove shrimp from the skillet and set aside.
In the same skillet, add bell peppers, snap peas, garlic, and ginger. Stir-fry for about 3-4 minutes until the vegetables are tender-crisp.
Return the cooked shrimp to the skillet and pour the sauce over the mixture. Stir well to combine and cook for an additional 2-3 minutes until the sauce thickens.
Stir in chopped green onions just before serving.
Serve hot over cooked rice or noodles.