You may create a Markup Item for 15% then add it to your Sales Order lines.
Also, to apply a markup item by percentage on a total order, add a subtotal item before the markup item on the Sales Order.
Here are the steps you may take:
A. Create a Markup Item
1. Go to Lists > Accounting > Items > New
2. On the New Item page, click Markup
3. On the Markup page, enter an identifier for the item in the Item Name/Number field
4. In the Rate field, enter the rate for this markup as either a percentage or a dollar amount. To indicate a percentage, type a % sign after the number. To enter a dollar amount, enter a positive number
5. On the Accounting subtab, select an Account
6. Click Save
B. Create a Subtotal
1. Go to Lists > Accounting > Items > New
2. On the New Item page, click Subtotal
3. Enter the Item Name/Number
4. Click Save
C. Create the Sales Order
1. Add the Items
2. Add the Subtotal line
3. Add the 15% Markup Item