How can I make money with 99 fletching?

One option is to buy (on the GE) unstrung magic longbows or shortbows and buy or harvest flax, spin the flax, string the bows, and sell them back. This will only make you about 160 gold per bow.

If you want to use magic and you already have a lot of money, you can buy, enchant, and sell onyx amulets for about 800K gp per amulet. I am not sure what the trade volume on that is, so it could be difficult to buy and sell the amulets.

If you have Lunar Magic and you have completed dream mentor, you can buy mahogany logs, use the spell Plank Make, and sell back the planks for a profit of about 800 each.