Harassment on a payment made

I am absolutely disgusted with Lancet's accounts department. I had a pap smear done at Dischem in Sunninghill. I paid cash at Dischem which amounted to over R400. I still have the invoices and proof of payment. The nurse advised that Dischem would be sending the samples to Lancet and that Dischem would pay Lancet for the samples since that charge is included in my invoice which I had paid for. A few days after receiving the results I received calls and texts from Lancet saying I need to settle the amount for the samples. I explain as detailed above and offer to give them the nurses details to follow up since my contractual relationship was with Dischem. I'm utterly disgusted at how Lancet and Dischem do not create the necessary process to conduct checks and balances. If Dischem is delaying on payment then I can't be held liable as I've paid Dischem who in turn is meant to pay Lancet. Kindly improve your process to eliminate Lancet from harassing customers that have settled their bills. It's really annoying and unprofessional.