When using Optimus Prime Optimized Prototype, when do you, as the attacker, choose to "spend" a die result... or has that been defined yet?

• One reasonable assumption is that it should be done in the same time frame as modifying offensive dice results. Roll your attack, do your mods, then decide whether or not to eat a shield. But spending a die result is not a modification, so is this the right time?
• Another possibility is that it works like Wampa, who (thanks to his FAQ clarification) cancels dice results in the Compare Results step. The sticking point here... is spending a die result the same as cancelling it?

You spend dice results during the modify dice step.

From the rules insert of the Resistance Bomber

Quote

Spent Dice Results

Some card abilities allow dice results to be spent. After a result is spent, the die containing it is removed from the rolled dice. A removed die is no longer part of the roll and does not contribute any other effects. Spending dice results is not a dice modification.
When attacking or defending, dice results are spent during the "Modify Dice" step unless specified otherwise. For other effects, the time at which the die result is spent is specified by the source of the ability.

Also in the Reaper rules.

So just to further clarify, when you spend your die result, the defender immediately loses a shield?

Correct. The shield is lose during the Modify Dice step, which means it's gone before they take damage.

For the purposes of Quickdraw's effect though, it's still happening during the attack, and the effect is still not resolved until after the attack.