Thanks for including that link. Reading through it, I'd like to clarify what you're looking for with QuickBooks Online. In this thread, we've been discussing being able to send a single email to encompass multiple invoices for a single customer. With the link you've given, it seems you may be looking to add multiple unbilled items to an invoice. If that's the case, that's already something that QuickBooks Online can handle with the Plus-level subscription.
When you create a transaction with billable time or costs and assign that to a customer, you can create invoices for the customer in a few ways. One would be to create it directly from the billable transaction, but another would be to go through the +New icon, choose Invoice, select the Customer, and then you'll see a sidebar populate with the unbilled costs you can add to the single invoice. It would look something like this.
If I were to click Add or Add All, the expenses would be added to the invoice. Even costs that you've specifically created from projects will show up in that side pane and will still be allocated to the project if you include it with other billable expenses. Here are some articles about the Projects feature and billable expenses if you're interested.
Let me know if you have more questions about that. I want to make sure we're on the same page with what you're searching for and what's available in QuickBooks Online.