I use CryptoWoo plugin for BTC payments and I use KyberSwap WooCommerce plugin for erc20 payments, you can select if you want to convert to DAI at checkout. BTC I just take the risk and believe it will keep appreciating in value over time.
We've processed over $200k in orders with over $100k being in BTC and ETH payments since 2017. Crypto merchandise website.
I don't use crypto for business and don't store much money this way (which would mean I have to plan) so far so I ignore it and I'm Ok. In fact I have only been affected by it in the positive way: I had insufficient BitConins to buy what I needed (not illegal) and planned to buy more but the rate grew quickly and I found myself in possession of sufficient value before I had time to bother.
On the Ethereum blockchain, things are a little easier. You can always use a stabletoken like DAI.
On the BTC blockchain, this can be a real problem. The best one can do is to hope that the BTC price doesn't swing more than the equivalent fee amount of a credit card transaction to make it worthwhile.
If you are willing to keep some of your crypto on an exchange (but remember, it is only really your crypto if you have the keys), you can try to automate the swap between BTC<-> stablecoin to reduce the exposure to price swings.
We've processed over $200k in orders with over $100k being in BTC and ETH payments since 2017. Crypto merchandise website.