The scenario that does not represent an impulse purchase is: Melissa goes to get an oil change for her car but decides to save the $60 and ask her brother to do the oil change for free.
An impulse purchase is an unplanned purchase made on the spur of the moment, without much consideration or planning. In the first scenario, Melissa goes to get an oil change but then decides to have her car windows tinted for $300, which is an additional expense that she had not planned for. This is an impulse purchase.
In the second scenario, Melissa goes to get an oil change but then decides to spend $500 on new car stereo speakers, which is again an additional expense that she had not planned for. This is also an impulse purchase.
However, in the third scenario, Melissa decides to save the $60 and ask her brother to do the oil change for free. This decision does not involve any additional expense or impulse purchase. Instead, it is a planned decision to save money by getting the oil change done for free. Therefore, this scenario does not represent an impulse purchase.