I think you can only do that by changing Bitcoin to USDT instantly and holding that. You can't do anything else otherwise, except also exchanging BTC to your currency. Bitcoin is experiencing a lot of volatility, which yes, it could mean you can spend 0.001 on a drink today and 0.005 tomorrow. I would exchange BTC into fiat, only as much as I need, instantly when I receive it and hold the rest in a 'freezed' wallet, for example a paper one.