Personal exports - retail exports: How the scheme works: Evidence of export
Details of the sale are entered on the VAT refund document. This document must be certified at the point of exit from the EC and retained by the retailer or the refund company as evidence to support zero-rating. Retailers who use a refund company may allow the evidence to be retained by the refund company on the basis that forms required by VAT assurance officers will be provided when required. Where refund companies do retain the original refund forms stamped by Customs, they may forward photocopies or electronic copies of the forms to the retailer. But the original form must be available on request, subject to the usual record- keeping requirements. This is because evidence of tampering is really only apparent from examining paper originals (see VATRES5050).
You should not, therefore, accept storage of original paper documents on optical media such as CD-Rom.