There are two scenarios you can consider doing to disable the 30% markup. Let me provide them to you and help you from there.

You can disable the price rules feature on the Settings. This can affect the conditions you’ve set based on customer, item, sales rep, and class. To do so, follow the steps below:

1. Go to the Gear icon, then select Account and settings.
2. Navigate to Sales tab, then click Products and services.
3. On the Turn on price rules, switch it off.
4. Tap Save, then Done.

Additionally, you have the option to edit items manually. QuickBooks Online makes changing this easy, and I can walk you through the process.

Here’s how:

1. Go the Sales tab, then select Products and services.
2. Locate the item you’ll want to adjust the markup and click Edit.
3. Scroll down to the Price rules section, then click the rule name. 
4. Make necessary changes, then hit Update rule.
5. Tap Save.

Once done, check the item again if the rule you’ve edited reflected correctly. This way, we can ensure you'll have the correct sales tax adjustment and markup per item.

See this article to learn more about controlling the price of your products and services: Set price rules in QuickBooks Online.

Keep me posted on how it goes. I'm here to ensure you get all the help you need. Have a great day.