I’d suggest you add your support for this feature request:

New Field Type: Nested Table

I build invoices with Airtable, but use a separate [Line Items] table to define each line that should be in the invoice, linking to those line item records from the record in the [Invoices] table. This can then be used to generate the invoice in Page Designer (which can build a grid based on those linked records) or via some other method (I build the whole thing as HTML using hidden fields and use Integromat to convert the HTML to a PDF file, which is then attached to the invoice record and emailed to the client).