Bad Service from Pick and Pay

We bought an iron for one of our cottages and it broke. I wanted to exchange the item and took a copy of the original invoice. I was however advised that the item will not be exchanged unless I produce the original invoice. The problem with this is that all these cash register invoices goes dull in a few months time - so we make copies of all of these and keep for guarantee purposes. Pick and Pay excuse seems frivolous. I phone customer care but to no avail - they have no answers on what will happen if the item breaks nd I produce an original that has become so dull that you cannot see what was printed.