When you begin to buy items for Infinity Edge, first sell one of the Doran's Blade, so you have room for more components and so you get some extra Gold

Jayce is also kinda squishy depending on what stance you're in. So if you focus on playing in Cannon stance, add some magic resist and armor. But if you focus on playing in Hammer stance, then you automatically get around 30-35 armor and magic resist, depending on the level of your ultimate

And also keep in mind that you should change stances often, for the simple reason that you gain speed and also bonus effects on your next attack. Cannon stance makes your next attack reduce the targets armor and magic resist for a couple of seconds, and Hammer stance makes your next attack do bonus damage. This damage and armor/magic resist bonus depends on what level your ultimate is currently in. The max bonus damage from hammer stance is 140.

And also keep in mind that Cannon stance = Trade defense for damage, and Hammer stance = Trade damage for defense. So try experimenting with the two stances for maximum effect.