Available only on certain plans
This feature is only available on certain plans and you may be required to upgrade your plan in order to use this feature.
One Time Charge allows you to specify a Customization that is only charged 1x per order of a specific item rather than per-quantity of that item.
Navigate to Products > Settings > Custom Options
Set "Enable Customization One Time Charges" to Yes
When Editing a Product, navigate to the "Customizations" section. There you will now see a setting called "How to Charge"
You can specify the customization to charge per item or One-Time.