How can I effectively track cost and sales of custom items?

Hello! I need to sell custom parts periodically and don’t want to add a new item every time. I’m mostly concerned about not being able to track cost and sales for accounting purposes. I think I came across a forum discussing this issue, but haven’t been able to find it again. I also use SOS inventory to tracks COGS and Sales to QBO, so I need something that could help communicate to both Shopify and SOS to track the special parts. Thanks!