The "Package files from other products" setting does allow you to choose one or more other products that already have their own file (regardless of whether the file is uploaded to us or remotely-hosted), so purchasing the "Package product" would issue links for the selected products' files.
Also, for remotely-hosted downloads you may find it helpful to know that we minimize transfer-bandwidth usage by caching a copy of remote product files. After the first successful download of a remotely-hosted product file, we retain our cached copy of that download in our Amazon S3 provisions; for every subsequent download for that product, we first check to see if the original copy at your URL has changed, and if not, we simply serve our cached copy from S3 to the buyer. This eliminates unnecessary transfer-bandwidth surcharges or limits for both you and us, and ensures the quickest and most trouble-free download possible for the recipient.
We don't charge as much for this cached-copy storage as we would for actually storing the files as uploads, as the latter case would oblige us to retain the files indefinitely as long as you have an active subscription, whereas we expect to be able to obtain remotely-hosted files from your file's original URL at any time, so we can optimize our cached storage needs by deleting file copies from the cache as needed.