No invoice/receipt for good purchased

I have been requesting invoice/ receipt for goods purchased more than a month ago. Despite several follow ups in writing and via the call centre and getting confirmations from agents Tanya and Sipho, Groupon still hasn't issued a invoice/receipt for goods bought. Not sure why their goods come with no receipts? Other online stores I have used e.g. Takealot, Zando, Spree issue document to confirm proof of purchase - Groupon does not?