Lancet admin is atrocious

Lancet Laboratories are used by some medical practices to analyse bloods taken. The invoices are submitted directly to medical aids, but if there is an outstanding amount not paid by the medical aid, Lancet sends an SMS similar to the following : "Mr ..........No payment has been received on your Lancet Laboratories (Drs Mauff AC & Partners) account. Please pay R******* to Nednank Acc Number .... and use Q .... as reference. For queries dial *** Email ***". If you contact them to ask them to provide the invoice, they either ignore you or promise to send the invoice, but NEVER do so. Who pays money to a company on a "please pay" SMS from a company, with no invoice being provided ?