There is no limitation for a free account uploading files to a folder shared by a paid account.
However, it is recommended using DriveHQ Group Account service, which is designed for such usage scenarios. The group account service is free and offers one sub-user license free. More user licenses are available at a very low price of $6/user/year.
All sub-users are treated as premium users. You can easily add/create/edit/delete/disable sub-users. If you have a lot of clients and if not all of them share files with you, you can easily edit the sub-username / password to re-use these sub-user licenses.
With group account service, your clients don't have to sign up DriveHQ themselves. Also you can test the features and help your clients by logging on as the sub-user account yourself. No DriveHQ support is needed in this case.