You seem to have a lot of insight about this problem. I wonder if it is more of either an addiction or self-medicating in conjunction with your mental illness rather than impulsivity.

You might consider an online support group for over spenders or debtors, especially if over spending is an ongoing issue for you.

Do consider returning things for refunds. If it is possible, but you just don't want to, that's another serious point to discuss with your therapist and/ or psychiatrist.