Pre-approval, done properly enough that sellers believe it.

A pre-approval is a lender's written assessment of what you can borrow, issued after your income, assets and credit have actually been reviewed. It matters because it's what turns you from someone who is looking into someone who can transact. In a market where a good house attracts several offers in a weekend, a seller choosing between them is weighing the risk that a buyer's financing collapses, and a properly underwritten pre-approval is the single strongest thing you can hand them to reduce that fear.

Why the Instant Letters Fail

The distinction worth understanding is between a pre-qualification and a pre-approval, because the words get used interchangeably and they're not the same thing. A pre-qualification is an estimate based on what you have told someone; nothing has been verified. A pre-approval means documents have been examined. Instant online letters mostly sit in the first category while using the language of the second, which is why experienced listing agents discount them. Getting yours done thoroughly also surfaces problems while they're still fixable - a credit reporting error, an unsourced deposit, a gap in employment history - rather than three weeks into a contract when the discovery becomes a crisis.