Request for Serial no. to be placed on invoice for iMac purchase

My daughter bought a Apple iMac from take alot, because iStore did not have stock. After numerous phone calls and emails they could not provide us with an invoice with the products serial no. on it, and I need the serial no. on the invoice for insurance purposes. It is unbelievable that in today's modern world, they just tell us that they can't do it, because they get the iMacs from another supplier. How is it possible to pay R46000.00 for a iMac and you can't even have the serial no. on the invoice. We have no proof that the invoice is for this specific iMac.