The Invoice and Quotes are related to both the Contact & Product. So, to sync Invoice or Quotes, the Contacts & Products need to be bidirectionally synced to keep both application updated. This will help you get the line item when you sync Invoice or Quotes.
Now, we do not have option to setup rules, please do write to us at support[at]smackcoders[dot]com. We will help you!
Thank you for using QuickBooks SuiteCRM Integration.
We haven't faced this issue before. Can you share us your SuiteCRM and PHP version? It will be helpful for us to replicate in our development server. If can, please do share us your staging SuiteCRM credentials at support[at]smackcoders[dot]com, we can take a quick look and help you get this fixed at the earliest.
We do have support only for the Products sync and doesn't support Service line item. And for the product line item sync issue, kindly share us your SuiteCRM credentials at support[at]smackcoders[dot]com, we will help you.