Let's get your inventory item quantities updated in QuickBooks Desktop (QBDT), @Peak Performance Coatings.

QBDT updates inventory quantities and value as you track every purchase and sale. When you receive the inventory (i.e., 2 cans of spray paint) with the bill, the quantity should automatically be reduced after creating an invoice directly from a sales order.

With this, I'd first recommend checking the items you used in your sales order and invoice. Once the items linked to the transactions are correct, you may have minor data issues with your company file. To fix this, I'd recommend utilizing the system's Rebuild and Verify Data built-in tools. Here's how:

1. Go to the File menu, then hover over Utilities and select Rebuild Data.
2. On the QuickBooks Information window, select OK. Follow the onscreen steps to save a backup.
3. Let the tool repair your file. 
4. When the tool finishes, select OK.
5. Go to the File menu. Then hover over Utilities and select Verify Data.
6. Let the tool check your file for data issues.

To know other recommend solutions for possible data issues with your company file, you can refer to this article: Fix data damage on your QuickBooks Desktop company file.

In case the quantities of your inventory items still show off after performing the steps above, you can manually adjust them to keep their status accurate. For the step-by-step guide, I'd recommend checking out this article: Adjust your inventory quantity or value in QuickBooks Desktop.

Then, I'd recommend pulling up the inventory report you need (i.e., Inventory Stock Status by Item) to monitor your items and their quantities accordingly. Go to the Reports menu and select  the Inventory option.

Additionally, to guide you further in managing your inventories along with the growth of your business using QBDT, I'd recommend checking out this article: QuickBooks Help Articles. It also includes topics about account management, banking, and expenses and vendors, to name a few. You can even explore video tutorials to learn your way around the product.

Please don't hesitate to keep me posted on how it goes in the comments below. If you have other concerns about managing your inventory items and sales transactions in QBDT, I'll gladly help. Take care, and I wish you continued success, @Peak Performance Coatings.