A baby formula company includes in the sealed box a coupon for 5 shekels off. Is one allowed to open the sealed container and then use the coupon therein to purchase that same container?
No, this is not permitted.
An item does not belong to a buyer before he “check out” at the counter, and it is not permitted to open the item while in the store, before paying for it.
Although some stores permit customers to eat before paying (see here), this will not apply to baby formula, and certainly not where the intention of the coupon is without doubt to encourage a future purchase, and not to give a saving on the current purchase.