How do I manage inventory for custom products?

Thanks for coming back to the Community, @mortiz1.
 

I suggest using our Bundle feature in QuickBooks Online (QBO). There you can manage multiple Kits to go together in creating a machine. You'll need to save the Kits in the inventory items before you can create a bundle.
 

Here's how to add inventory items:

1. Go to the Gear icon on the upper right and then select All Lists.
2. On the Lists, hit Products and Services.
3. Click New and then select Inventory.
4. Enter the data needed (Name, Quantity, Date, Cost).
5. Hit Save and close.

You can now include the Kits  in a Bundle, here are the steps:

1. Under Products and Services, click New.
   
2. Choose Bundle (enter the Name and the Kits/items).
   
   
3. Click Save and close.

You can also display bundle components when printing or sending transactions by checking the box.

You can use this video article in managing your inventory: Setting up Inventory in QuickBooks Online. It includes how to enable inventory tracking and how to enter purchase orders: recording, tracking, and more.
 

Should you have questions, leave a query to this thread for further assistance. I'm always here to help you. Enjoy your day and take care!
 

Alternatively, have you tried checking out the QuickBooks App Store for a tool that can help you manage your sub-assembles?
Katana Smart Manufacturing Software integrates with your QuickBooks Online account, so you can easily track and manage your multi-level bill of materials, while still being able to track your finances with QBs. WIth Katana you can:
Track your finished goods & material availability;
Manage your bill of materials (product recipes);
Priority-based production planning;
Multichannel sales support;
Manufacturing floor-level control; and
Manufacturing cost calculations.

Thank you for this explanation. Is it possible to adjust the price of the bundle from the cumulative prices of all the parts?  We bundle products and then provide a small discount.

Yes, it's possible to adjust the price of the bundle from the cumulative prices of all the parts, @betefleming.

You can change the price of each item to change the price of the bundle.

1. Go to the Sales menu and then select Product and Services.
2. Find the item, then click Edit from the Action column.
3. Change the price of the product.
4. Select Save and close.

Since you also wanted to include a discount, you can create a discount item and add it to the bundle. However, we still need to enter the negative sign under the Rate column when creating a transaction.

You can follow these steps:

1. On the left pane, go to Sales.
2. Choose Product and Services, choose either Non-inventory or Service. 
3. Fill up the name and rate (e.g. 10%) of the item and all the necessary information, under Income drop-down select Discount given.
   
4. Click Save and close.
5. Under Product and Services, click the bundle and add then select +Add lines to enter discount item along with other items.
   
6. Hit Save and close.

In the invoice window:

1. Under Product/Service, choose the bundle. 
2. Choose the affected discount account, then under the Rate column input the negative sign before the rate/decimal number.
   
3. Once done click Save/Save and close.

Alternatively, you can simply add the bundle and then put the discount item together to an invoice.

Additionally, you can reference this article for more details on which type of item can be changed: Change product and service item types in QuickBooks Online. This contains a brief insight into adjusting inventory items.

If you have other concerns in regards to changing products and services and create a bundle, just tag me in your comment. I can assist anytime when possible. Take care and enjoy your day!

@betefleming 

You may need to have an additional app to do so.