Customer would like to receive invoices that are in line with their split cost reporting, rather than broken down by account. They would like to be able to generate invoices that include % of shared costs like this have in this report.
Eg. https://console.doit.com/customers/GnnFMff9PzV9jH1UueMA/analytics/reports/IZ9C0e3zL44ki0PZ0dES

The only option for custom invoices is custom invoice buckets, which is still account-based meaning that accounts with split costs cannot be divided between different invoices.