Do you have Strike? You can pay a lightning/bitcoin invoice directly from your USD balance. On your end it just spends your fiat. The recipient receives sats or whatever other currency their wallet supports on the receiving end.

Spend and replace wallet is a great idea!

Strike

You directly (buy and) send Bitcoin. Just put fiat on it and send lightning (1ct per transaction) or onchain (from free fee to the high priority on mempool).

So you buy when zapping.

Cash App does this!