Bank of America (BAC) has close to 40 million customers who bank digitally. Around 30 million of which use the bank’s app and 10 million use its virtual assistant, Erica — a cornerstone of the bank’s digital strategy.

But that’s not enough for America’s second-largest bank by total assets ($2.4 trillion) — and perhaps most digitally advanced — bank. Starting in the second week of December, the bank will deposit $15 into the accounts of customers if they try mobile check deposit, transfer money to someone via the Zelle app, and use mobile bill pay by Jan. 17.

Customers can see if they’re eligible via the app, but will also be notified of eligibility if they try to deposit a check at an ATM or at a bank. The machine or a teller will inform them that they’ll be compensated if they do their transactions on their phones.

While many customers are younger with considerable experience with digital banking, Bank of America has plenty of customers who aren’t as comfortable with the latest banking technology and don’t yet know the full capabilities of a digital app. This promotion is a way of nudging them to step into the digital world a bit further.

Why pay people money to deposit checks?

For the bank, it’s also a simple question of math and unit economics. The app is cheaper to operate.

“Taking a check into a financial center and handing it to a teller is more expensive than putting it into an ATM,” said David Tyrie, Bank of America’s head of Advanced Solutions and Digital Banking. “And depositing a check in an ATM machine costs more than mobile check deposit.”

The bank began mobile check deposit technology back in 2012 — USAA debuted the tech in 2009 — after seeing that it could dramatically shorten teller lines if it found another way for bank customers to deposit checks. Since then it’s become a standard feature for mobile banking apps.

The Charlotte, N.C.-based bank is currently testing paying customers to deposit checks and use its other digital offerings with a million users who haven’t yet used all the functionality of the app. For Bank of America, engagement is key to its digital strategy, which is centered on its Alexa-like chatbot Erica, customization and personalization, and smoother interactions.

“It’s one thing to be focused on how many digital users you have, but you will eventually get to a stage where it’s now not how many users you have but what kind of engagement,” said Tyrie.

Tyrie’s team wants people to do more than simply look up their account balance, but to actually bank from the app itself when possible. A lot of things are really easy and don’t require much in the way of hand holding. But Tyrie is quick to note that there’s always the choice of having a high-touch approach with a human if necessary by walking into one of the 4,300 physical banking branches.