Cash is the only truly anonymous method of payment. Bitcoin has a ledger that will be around forever. Eventually computer forensics will be able to use that ledger to peg transactions to IP addresses or possibly trace it straight to the user with the help of ISPs. If you really want to be anonymous when you buy online I suggest Simon Gift Cards and Debit Cards.
https://www.simon.com/giftcard/How can cash be anonymous, and Bitcoin not so? If I am using a proxy server or a VPN or other anonymization services such as the I2P browser, how can someone find out my real IP address? And if I am using a one-time Bitcoin address, created just for a single Bitcoin transaction, I don't think anyone will be able to trace me using it.