We use Replicon for all of our client invoicing. In order to add VAT to an invoice, we need to manually add a line for VAT and manually calculate as 20%. This is cumbersome and manual and can lead to mistakes, especially if the invoice changes. Really and invoicing system should be able to handle a Sales Tax easily. I would suggest each Client or Project should have a Tax Code and this automatically gets added to invoices when you add a time, expense of manual line to the invoice.