Please follow the steps mentioned below to show the percentage of the additional charge:
Step 1: Go to Sales dashboard >> Document settings
Step 2: Click on Add custom fields
Step 3: Click on Additional charges >> Click on Edit for the charge which you want to make changes
Step 4: Turn on Show percentage(%) for additional charge toggle >> Click on Save
Step 5: Click on Update settings
Step 6: Now you can enter the percentage of the additional charge in the invoice >> Click on save
Step 7: Now the percentage of the additional charge will be shown in the invoice as shown below.
Note: This feature is applicable only while using Evergreen template