A good practice is to deliver a slightly incomplete, lo-res (MP3), or watermarked file, prior to final delivery of hi-res files. This way you can get confirmation from the client that everything is in order creatively, and from a production standpoint. If you have the client's approval, prior to your final delivery, then there should be little reason to contest your final delivered work.
If you don't offer a 100% guarantee, clients cannot receive a refund if the service was delivered according to the service terms.
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