It is poor English.
They have put two instructions/notices to customers in the one sentence.
Try reading it thus with added guidance.
£5 reward to customers who do not receive a receipt ......
Tell Management the checkout operator has failed to give you a receipt to claim your fiverPlease retain receipt for proof of purchase.......
We will not change the item, no matter how faulty if you don't save your receipt, if you haven't been given one ask for it now and also claim a £5.YES I know "if that's what they mean that's what they should say" but perhaps they are crediting their customers with having possession of a modicum of common sense. Foolish of them but I expect they live in hope.