I understand fully, though not on such a large scale as 3/4 of an entire side of a car, how frustrating a bad respray can be...My hat's off to you sir. You did an amazing job.
I agree with several of the above, it's nice to aim for perfection, but sometimes, it's just not possible.
Returning some of the money was an incredible gesture on your part. I hope your customer appreciated it and will pay it forward in the future through loyalty and referrals.

I'd do it in a heartbeat if I thought my work wasn't up to par in relation to the money I was charging, so good on you man.
