It's not possible to import invoices unless you have a Customer Reference. However the Customer Reference field is not madatory in the software. Therefore you have no way to import unless you manually create a customer reference for 100s of customers records, which you have to do one at a time. The same challenge applies for importing supplier invoices.
There needs to a be a better way, perhaps by being able to import using the Customer/Supplier Name or by providing an automatic ID for customers and suppliers in the software, and allowing the import to use this ID instead.