Only the direct deposit payroll account's primary principal is recognized to request a limit increase. This is due to the security of your client's account and because there might be additional documentation (i.e., bank statement) that needs to be submitted. Their request will be reviewed within 2 business days. Then, they'll receive an email notification regarding the result.
To learn more about this, I'd recommend checking out this article: Direct Deposit Limit. It also includes details about the options to pay your client's employees.
Also, you can refer to this article if you wish to learn more about how the direct deposit service works using QuickBooks Desktop Payroll: Direct Deposit service agreement. It includes details about activation, limitations, and your responsibilities, to name a few.
Keep me posted on how it goes in the comments below. If you have other payroll concerns or inquiries about the direct deposit in QuickBooks, I'm just around to help. Take care always.