What is the best way to ship nationally a package which is 2kg and the size is 50cm x 30cm x 5cm? I have about 700 of those packages to post

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What is the best way to ship nationally a package which is 2kg and the size of the package is 50cm x 30cm x 5cm? I have about 700 of those packages to post, so looking for the cheapest way to post them all. Any discounts for bulk shipping?