Currently, the option of auto save when creating an invoice or estimate are unavailable in QuickBooks Online.
I understand the importance of this features to your business. Rest assured, I'll personally send this feedback to our product developers. This way, it might be given a chance to be added to our next products update.
I suggest visiting our Blog site. This is where we share recent happenings and future developments, such as updates to newly added features. Here's the link:The QuickBooks Blog.