FHP International Corp., a Fountain Valley-based health maintenance organization, said its self-tender offer for 2.5 million shares of its common stock outstanding--about 20% of the shares now on the market--has been successfully concluded. The company said it will spend up to $16.9 million to repurchase the stock, or about 12.5% more than the current open market price for FHP shares. Noting that its offer expired at midnight Wednesday, FHP reported that about 2,572,503 million shares were tendered to the company. The company said it would take at least five days to determine the precise number of shares it will buy and the final price it will pay for the stock. But based on preliminary estimates, the company said it expects to pay its maximum offer of $6.75 per share. FHP closed Thursday at $5.875 a share in over-the-counter trading, down 12 1/2 cents for the day. FHP is one of about 20 county companies that began buying back their own stock at discount prices following the Oct. 19 stock market collapse.