When you create a bid package, you will need to attach all of your bid documents in one folder in the Documents tab (e.g. ‘Bid Documents’). Underneath this folder, you can create unlimited subfolders for each bid package and upload the relevant documents into each. (Note: Only folders and sub folders that have files in them will be available to download using the download link in the invitation email.)
Your folder does not need to be public (i.e. it can be marked "Private" and only available to 'Admin' level users and those you specify). You will not see an error if you try to attach a private folder to a bid package.