At this time there are no limits on the number of transactions, the attachments that can be made to those transactions, or the number of users.
To date we've found all users are reasonable in their usage of data.
We do reserve the right to charge extra or close accounts that in our opinion abuse this, but you'd have to be trying on purpose, so just don't, eh?
Kidding aside, the data for a set of books for a company often smaller than one JPG photograph. We do have some concerns about attachments, but so far everyone's been very reasonable.
Thank you so much!