Hi there, smleick4484.
I'm by no means an accounting professional, but I'd like to join this discussion and add to Rustler's awesome answer.
Yes, you can post the Service items to the same income account assigned to your Inventory items. With this, you shouldn't need to make any other adjustments or journal entries in QuickBooks Online.
However, please be sure to reach out to an accounting consultant to ensure this method is the best suit for your business. With Intuit's Find-A-ProAdvisor site, you can search for certified professionals in proximity to your ZIP code that can discuss options more personal to your business's needs.
That should get you going in the right direction. Please let me know if there's anything I can help you succeed with. I assure you that I'll do my best to help you out!