Bitcoin can be used as currency but also as just a payment protocol. Many shops interface with Bitpay or Coinbase in order to accept Bitcoin-as-a-protocol: The payments are instantly converted to fiat and the shop neither holds Bitcoin at any moment nor is it exposed to exchange rate fluctuations.

On the other side of the deal, i.e. if I as a customer want to pay by Bitcoin-as-a-protocol without currency exchange risk, there is no service I can use, is there? Of course I can keep fiat at an exchange, then trade the needed amount into Bitcoin and send it to pay for my order. But it seems quite cumbersome.

I imagine a wallet provider where people could deposit fiat into their Fiat Wallet and use a browser-plugin (or something) to easily pay with Bitcoin-as-a-protocol. Like with KryptoKit you could pay a Bitcoin invoice with two clicks and the sent amount of Bitcoin is deducted from your fiat balance by current exchange rate.

This concept has been on my mind for quite some time, but so far I haven't even seen anyone talk about it. Does a service like this exist? Is it a reasonable idea in the first place?