Products that use any of the following settings are not available for updates:
-Send Stored/Generated Codes;
-Send Transaction Data to a URL / Payment Variable Information URL (this setting is used with PDF stamping);
-Remote Product File URL.
If your product is an ebook it may simply be that you have PDF stamping enabled, in which case you can easily make it available for the update service by temporarily unchecking the Send Transaction Data to a URL setting. When sending updates there are too many links being generated in too short of a time for us to perform functions like applying stamps to each file or calling up outside addresses link by link.
If you are hosting an ebook remotely you should probably be able to upload it directly to your account's local storage, even if just temporarily, since individual ebooks are not usually very large.