Your information is partially correct. In order to be reimbursed for the fabric you would have to supply proof, meaning all your receipts. Do you have receipts for the fabric? I don't believe the insurer will take your receipt for custom quilting as a fair value should the worst happen. To my knowledge the only thing an insurer will take other than receipts is a certified appraisal. And even then, you may wish to insure the quilt independently, not use the insurance the postal or freight carrier offers.
I would recommend you find a certified quilt appraiser (AQS certifies appraisers) You will have to pay for an appraisal of the quilts value. Make sure the appraisal will suffice as a value for insurance loss purposes. That would be your best bet. A certified appraisal will probably run you over $100.