New items are created on the "Items" tab on the Settings layout. You can create items for physical things, like products you sell, or for intangible things like services you offer.
Begin by adding a Quantity on Hand for each of the items you'd like to inventory: and check "track qty on hand" for these items. Also enter a re-order quantity: when inventory falls below this level, item quantities will turn orange on this Settings screen and on invoices containing this item.
Once this is set up, invoices containing an item for which "track qty on hand" is turned on will display an "update inventory" button. As inventories items are added to the invoice, we lookup the current qty on hand. After "update inventory" is clicked, we write the new qty on hand into the item's record on the settings screen and pull the new qty on hand down into the invoice line item: this way we never debit the same invoice line from inventory twice.
Changed to the items' quantity on hand are transactional, so if the item is being edited by another user, or another order's "update" script, the changes are reverted and users told to try updating again in a moment.
One advantage of this set up is that the qty on hand in the items record is indexed, so it doesn't matter how many items you have, nor how many invoices you have: the system stays fast.
Caveats
This is a solid, but simple way to track inventory levels. There are a few things you may want to change or extend to make this into a more sophisticated inventory system:
Yes. You can bill appointments by selecting the "invoice" tab on the Event Details mini window (this is the window the comes up when you click on an event in the calendar). Lines created here are added to a new invoice linked to this event. That invoice is also linked to the event's contact and project.
Once lines are added you can close the event window and work on the invoice later, or you can jump right to the invoice by clicking on the green arrow pointing to "Invoice" on the Event Details window.
You can take this idea (and the scripts used here) and extend this to bill other entities like time. This takes a bit of work, but two scripts in particular should help...
One an invoice is created you can add lines to it from other contexts (like time) using the script "Create Invoice Line { InvoiceID , ItemID , Qty , Name , PriceEa, Desc , Taxable , Override }"
That is your real worker script to make new invoice lines for an existing invoice.
And "New Invoice" is the basic script for creating a new invoice.