This should be simple enough, but I can't find an answer anywhere

We're an atelier that makes specialized clothing, but it doesn't make sense to charge by the hour because different projects require different specialists and processes. We can add a line for the BOM, and also a line for other fixed costs, but why do I have to make a new product every time I make a new invoice? It clutters the product page and it makes it rather cumbersome to add a product each order.

Also we charge for services that are exclusive to that order. IE "Darting on left sleeve" "tapering of pant leg". It doesn't make too much sense to add these as individual products or services because they'd be a different cost each time.