For one-off charges, you can adjust an invoice; in most cases, you'll want to create standard fees that you can apply to any invoice, in a couple of clicks. Fees can be fixed (a specific dollar amount) or proportional (a percentage of the invoice's subtotal). And your fees can be applied before or after your sales taxes / VAT.
Create a taxable fee
Go to Money › Discounts & Fees, and click on the "New Discount / Fee" button.
From the "Type" dropdown, select whether this fee is fixed (always the same dollar amount, regardless of invoice subtotal), or proportional (a percentage of the invoice's subtotal). Type a name for the fee, and the amount or percentage. Optionally enter a description (only visible by your staff), and click Save.
Create an after-tax fee
The typical example of an after-tax fee is a late fee or finance charge. A late fee is typically fixed (example: a $25 late fee), while a finance charge is often proportional (example: a 10% finance charge). But all this is up to you and your agency's service terms. Depending on the laws in your area, set-up fees might be non-taxable.
Apply a fee to an invoice
You can apply fees while you are creating the invoice (while it's still a draft) - an example would be something like a TypeWell set-up fee. You can also add fees to already published invoices: a late fee is a typical example. Either way, scroll to the bottom of the invoice, click on the "Apply a fee" button, and select the fee you'd like to apply. Terpsy will apply the fee, recalculate the invoice's subtotal and balance, and auto-save the invoice. Customers will see the fee when they next open this invoice in their Account Manager.