Prepare the Slow Cooker Sauce Base: Pour the entire contents of the bottle of BBQ sauce into the basin of a large 6-quart or 7-quart slow cooker (Crockpot). Ensure the sauce covers the bottom surface evenly.

Add the Cranberry Element: Scoop the full can of whole berry cranberry sauce (do not drain) into the slow cooker on top of the BBQ sauce. Use a rubber spatula to ensure all the cranberry sauce is added.

Incorporate the Remaining Flavorings: Add the brown sugar, apple cider vinegar, Worcestershire sauce, smoked paprika, and the optional crushed red pepper flakes to the sauce mixture in the slow cooker.

Whisk the Sauce Thoroughly: Using a sturdy whisk, combine all the sauce ingredients until the mixture is uniform and the brown sugar has mostly dissolved into the liquid. It is okay if small lumps of cranberry sauce remain, as they will melt down during the cooking process.

Add the Meatballs: Open both bags of frozen meatballs and pour the full amount directly into the slow cooker. Do not thaw the meatballs before adding them; they should be placed in the crockpot while still frozen.

Coat the Meatballs: Gently stir the meatballs with a large spoon or spatula, ensuring that every meatball is completely coated with the cranberry BBQ sauce. Make sure none of the meatballs are left sitting dry on the surface.

Begin the Cooking Process (Two Options): Place the lid securely onto the slow cooker. Cook on HIGH for 3 to 4 hours, or on LOW for 6 to 8 hours, until the meatballs are thoroughly heated through and the sauce is bubbling hot. Stir occasionally during the last hour if possible.

Serve: Once cooking is complete, give the meatballs a final thorough stir. Switch the slow cooker to the 'Keep Warm' setting for serving. Serve immediately with toothpicks as an appetizer, or over rice or mashed potatoes for a full meal.