So grouping them is the only way? Combining the invoices in one line without Summary will not work? I guess so but I'm kinda hoping that it could be possible with some formulas.

Are you looking at concatenating maybe? You could concat all of the invoice document numbers related to a sales order. I have a saved search that does this exact thing. It will show the sales order, and then all of the related invoice document #s on one row. In the criteria I have Type = Sales Order, Main line = false, Billing Transaction fields... Type = Invoice. Then in the results I have the document number grouped, and then a Formula (Text) field, with the Minimum Summary. The formula is ns_concat(distinct({billingtransaction.number}))

Yes yes. Thank you so much!