You can already generate a P&L for any period you define under Reports.
I am guessing that what you refer to when you describe a P&L for an individual sales invoice is really project accounting. Manager does not have that capability yet. It is complex to do in any meaningful way because it is difficult to properly allocate expenses such as rent, utilities, fringe benefits, etc., across multiple projects. It is common to add those types of overhead and general and administrative expenses to base labor rates and/or material costs as percentages. But even when it is done, it is frequently done in a separate project accounting program, not the business’ main accounting records.
When you describe using only inventory sales and inventory cost figures, that really isn’t a profit and loss statement. If I understand what you mean, it is a calculation of gross margin on inventory sales. That’s what you get with the Inventory Profit Margin report. You might look at what information you can recover from that report with clever definition of time frames and by drilling down on appropriate figures within it.