Hi there,
Syncing third-party apps with QuickBooks Online is a great way to expand your options for your business bookkeeping. I know how important it is to have all of your invoices appearing in your account. I'll point you in the right direction for help with this.
Any questions regarding the process of bringing information over to QuickBooks from a third-party app will have to go through the app's support team directly. I've found the website for Kashoo, and it looks like they have their own version of a help forum here: Kashoo - Advice and answers from the Kashoo Team.
I recommend posting your question there.
If there's any other users here in the Community who may be able to offer advice regarding Kashoo and QuickBooks Online integrations, I'm sure they'll reach out.
Have a great day!