If you think of it, that was easy~ Offchain.
Use or partner with a 3rd party service (Ex. Coins) that accepts bitcoin and can send and receive bitcoins between users without a fee (or just their service fee, they didn't send the funds using the blockchain anyways).
Or establish a 3rd party wallet accounts (offchain) to be used for the site that can hold (pseudo) btc balances from users to be used for purchasing products.
With this, you can even offer refunds which is impossible using the blockchain.
The process is Centralized, but micro-payments needs centralization.
Take note of the security vulnerabilities.

The only drawback is, users have to use that particular service in order to pay for your store or establishment.
Just add a different payment option and you're good to go.