Hi. I own a commercial cleaning co. I want to add supplies purchased as inventory then expense supplies (non billable) to customers as I use them. How do I do this?

you receive and stock inventory you use just as any other inventory type item.  You must have a subscription to QBO plus to have inventory items available

then inventory only gets used against a customer/job in one of two ways

you put the items/qty used on an invoice, they can be listed at a zero sales price

or

you use inventory adjust, set the adjusting account to COGS and lower the qty.

gear>settings>company settings
sales>products & services
and turn on qty tracking (inventory)
and
in company settings you turn on the items table and purchase orders.

Then when you enter an expense or bill, about half way down the left side is a title "item Details" click that and a table opens where you can select the item, qty and item total cost for purchasing inventory