Can I add an additional tax for certain customer/resellers? I understand I cannot have multiple taxes on a line item but is there a way to add tax on subtotal?

Thanks for reaching out, @carson-lupulorex.

At this time, being able to add tax on subtotal is not an option in QuickBooks Online (QBO). I recommend sending this request straight to our product engineers through feedback. Sharing features and options that you would like implemented is how our engineers look for new product updates.

To send feedback, follow the below steps:

1. Go to the Gear icon at the top.
2. Select Feedback.
3. Enter your comments or product suggestions. 
4. Then select Next to submit feedback.

For future reference, adding you some articles which you may find helpful:

• How do I record tax withheld by my customer?
• Create a custom tax rate

If you have any questions, you are welcome to ask here in the Community. I'd be glad to help.