If you buy a lot of fish and plants, the shipping cost is spread over more items and works out less per fish. If you only buy a couple of fish, the shipping cost will hit you hard.
We had this issue when we imported fish from anywhere. It cost use several thousand dollars just to have the quarantine department look in the boxes, and the airlines charged us per box, regardless of how many fish were in the box. So we tried to get as many fish in each box as possible.
If we had 10 fish in a box, it cost us the same as if we had 500 fish in a box, at least from the freight and import duty/ charges. However, the price was then spread out over 500 fish instead of just 10.
Normally we would bring in 10,000+ fish per shipment, and the amount of money in freight and charges only increased the total fish price by a few cents per fish.
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Never buy gravel or rocks online unless they have free shipping for it. The heavier the item, the more it costs in freight.