To add a standard fee to an inventory item, you will want to start by opening the system (gear) menu and selecting "Inventory Settings".
Once here, you can filter to the inventory you are looking for. As an example "Parv" was provided as a filter:
Once you find it, click on the item you are looking for. For example "Parvosol". This brings up the inventory detail popup:
Next you click on the "Price Calculation Settings" tab, where you will see the "Dispensing Fee". Fill this out with the fee you wish to add and click save.
All new usages from this point forward will include this fee with this inventory.