Ah, another math nerd that isn't asking what the item is or how much it costs to ship back defective items.
There's no such thing as mass produced items that don't have some percentage of defects. Like, what's the insurance policy on the items? Who pays for return shipping when defects are returned?...
There's more to it than a pocket calculator can answer.
Returns and defects can effect savings per item, but the question does not include a clause that requires this calculation. Since the question does not mention it, it can be assumed OP was asking about savings before any defects or returns are made. This is not a net savings question, it is a gross savings question. Gross savings of course being the savings before any modifiers such as sales or import taxes (which are also not mentioned and therefore rightfully ignored by the person you attempted to "correct"), or in this case, defects and returns.
If it's food, it needs constant refrigeration and such. If it's fidget spinners or whatever, those can usually last on the shelf for years.
Without knowing even roughly what OP is looking to purchase in bulk, we don't have sufficient information to calculate.
Let's say you purchase 1000 bananas. How long are they gonna last before they're all sold, vs going bad? But if you purchase 1000 fidget spinners, they might last 20 years on the shelf.
Insufficient information. Without knowing even the type of product, we can't make any assessments on the additional cost of storage.