When adding a service as an admin in the admin UI, or ordering one from the order form, the price of the generated invoice differs from the totals shown during the add process. This accurs when a coupon is applied to the service.
When applying a coupon to the service, it appears that the totals are calculated by taxing the coupon amount, but when the invoice is generated, the coupon line items are not taxed, so the price is higher on the invoice than what was shown in the totals box when adding the service.
Additionally, it appears that applying tax to the coupon line items on the invoice after the fact will reduce the total price, but it reduces it a little too much. I suspect this discrepancy may be caused by rounding afterward, and this will probably go away if the invoice taxes the coupon amounts at generation.