Does this sale fall under the category of salam (forward) sale and is it permissible, given that the purchase takes place after opening the letter of credit and allowing a 45-day period for preparing the shipment, then the fish are bought, prepared, and shipped with the seller's money after the buyer's approval?

The transaction mentioned is not a salam sale, because the seller does not receive the price in the contract session, which is what distinguishes a salam sale. The presented scenario is that the seller shows the buyer the specifications and prices of the goods. If the buyer agrees, he opens a letter of credit which the seller does not receive until after the goods are shipped. This makes the sale a sale of an absent, described item. This sale is permissible if it occurs after the seller has acquired ownership of the sold item (the fish), and the preceding procedures were a promise to sell and documentation procedures, not a sales contract, because selling something before owning it is void.