is the price of BTC locked in when I initiate the transaction?

If I’m buying bitcoin through my Coinbase account with an ACH payment from my bank, is the price of BTC locked in when I initiate the transaction? Or 3 to 5 days later when the ACH payment arrives in my Coinbase account? Is there a faster way to buy btc or eth on coinbase using USD from my bank than the 3-5 day wait for the funds for transfer without using a debit card with it’s high fees? Not loving the idea of keeping USD in my fiat wallet in my coinbase acct, since I see conflicting info about whether or not it’s insured.