how to buy bitcoin with the intent to buy it once, pay trough a qr and never use it again

Without going in to detail and saying any personal information about myself, i need to get a really small portion of a bitcoin quickly and pay with it immediatly. I searched all troughout the internet and there is so much conflating information and also most guides focus on people that want long term investment. I would also preffer if the site i am buying the bitcoin on does not require an id or any other piece of documentation.