@ Mike C

"Tax Invoice" implies that there is VAT involved. A client who can claim VAT back will not be able to claim if they receive a "Cash Invoice". SARS is very clear on this point.
Perhaps I wasnt clear enough, bot are "Tax Invoices" however one is a Credit Sale and the other a Cash Sale, and the relevant description should be shown on the document

Not unless you are buying something via Game or Incredible Connection etc, they will give you a Pro-Forma which can either be ignored if you are not going to buy it or changed if you change your mind to another product. A pro-Forma will be given if you would like to claim from Insurance for example. Once again a non binding or non legal document between yourself and the company giving it in the sense they cannot hold you to it.
You would find the shipping Industry to be very very different, I dont think small purchase such as from Game would warrant the unnecessary additional paper-work, as I mentioned previously there must be good reason for a Proforma Invoice, otherwise, it begs the question why the extra hassle?