There are certain details (client name, date, brief line item and amount) that have to be filled in to create an invoice in Crunch, whether just for bookkeeping or sending to your client, but you can keep this quite simple.
Using the reference field or the notes section to add the invoice number from your other system is a very good way to keep track of what’s.
If you have other projects that aren’t invoiced elsewhere, you can create these in Crunch and send them directly to your client (if needed).
All the best,
Katie