I like the argument about cause and effect being an indicator of no free will.

People make decisions based upon a variety of factors and every input of a new factor changes that decision to a certain degree, no matter how off the wall that factor may be. You can try to fuck with the end decision as much as you want but every new input is still a defining element of your decision.

Imagine a scenario like the OP explained where the subject is thinking about purchasing a bike. Childhood memories are clearly a factor, as are ease of travel, thrill-riding, and exercise. Those inputs are all internal, within her mind. Externally, inputs might be that it's wonderful weather for a ride, bike sales are on, or her friends are into it.

Regardless of of the quantity, variety, or type of inputs, they all determine her decision to some degree. Even if she has the ability of foreknowledge and can "know" her final decision, she is still determining her decision through another input. And even if she tries to change that decision, that change is inspired by the foreknowledge (cause) which is another input.

It's impossible to act arbitrarily because - as said earlier - any attempt to fuck with "fate" or the established flow of time takes conscious thought: the impulse to act. Coming up with an original idea that might interrupt time flow - like swinging my hand for no reason - is still caused by a reason: the desire to screw with pre-established order.