I have 4 invoices that are ordered every first day of the week. At the end of the month, I want to send 1 invoice to the client. How can I merge the line items of these 4 invoices to 1 invoice?
Not by default. You will need create the functionality.
One way to solve might be by creating a custom module that collects information of the required invoices (via custom related fields) and then creating a after/before logic hook to create a new invoice from the related invoices.
Here it might be useful the
Collection of fields created by “p.konetskiy” to collect the invoices’ in one field