Hey. While we're building a slight competitor to Dust, I can give you a partial run down on how it works.
Dust, along with his(Cobain) main competitors, Crypton & CoinFlash leverage Coinbase as the wallet / account / exchange structure. Then, with the integration of a third party provider, Plaid, these apps can login to your account, view/collect data, in order to facilitate the transaction. Plaid is also a clearinghouse to process transactions as Coinbase is merely the exchange / wallet.
Dust doesn't(shouldn't) collect or store your banking information. Not your login credentials, anything. Just access to review financial data.
https://plaid.comCoinbase already has deep integration baked in with plaid now, but these apps have to create their own account. While Cobain eluded to develop beyond Coinbase, it hasn't been stated explicitly anywhere.
Hoard @HoardInvest -
https://hoardinvest.com offers something unique. We bypassed Coinbase altogether, bypassed plaid altogether, and began developing our own decentralized exchange and multi-SIG multi-currency wallet explicitly to offer more than 3 coins, waaay more than 3. So with Hoatd, you can get those Alts on the cheap.
Not sure what he charges end users. I can guarantee we're cheaper. Also, because we skirted Coinbase, we face US FinReg requirements. Licensing that's expensive. But that puts us on a whole other level as has as capabilities, fees, reach.