Actually, it does not have to be from the same seller to get the discount, but the DROS system has issues with multiple handgun purchases, so typically the FFL would have to call the DROS people to get the correct amount to be charged.
For long guns, it is not possible to have two PPTs from different sellers on the same DROS since the seller's information is required. This means it might be cheaper to do it as a dealer sale, although then the fee would not be limited, so that needs to be factored into it.
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