Hello

You can create your invoice without using product and bydefault incoive not picking product qty only take 1 but not move stock

so you can create invoice without using product

just mentioned in description then select the default qty (1) and unit price then update

You can create a financial invoice that does not have a product code - line value is a function of quantity*price so you will need a unit of 1 at least. You can use analytic accounting to link to the project.