Before you will be authorized to upload for your client, their quote needs to be set up to accept the upload.
The customer needs to complete their project by selecting an option which allows you to upload on their behalf. (Third Party Upload Option) You will then be emailed instructions about where to upload and the criteria to use (such as account number, project number, templates, etc).
Since there is an additional fee associated with this process you will not be able to authorize this on their behalf.