First, you cannot add amounts to a subtotal except through the tax code feature, so only taxes.
Second, Manager will not perform calculations other than those hard-coded into the various forms, working from data input on the line items.
So miscellaneous charges like these must be added as line items. They can be defined as non-inventory items with pre-established posting accounts, default charges (that could be modified), etc. But percentage amounts would have to be calculated first and entered. Which accounts you post them to is up to you.
One of the problems with the approach you illustrated is that under double-entry accounting, you would be debiting the extra charges to
Accounts receivable. You need an account for the balancing credit, as well. That means that such charges require more than an entry into a custom field, even if a field could perform arithmetic. Some such charges are even taxable in some jurisdictions, so it isn’t a matter of just adding on some extras.
So, assuming these are charges you impose, you might want to set up income accounts for freight and packaging/forwarding revenue. Freight charges from others that you pass along have to be handled differently, of course.