Invoicing software

I am currently using an add on in our accounting package to produce, number and post sales invoices. This add on is rather cluncky and difficult to use.

Could anyone recommend some invoicing software that will allow me to take data from possibly csv or xml format, and automatically create invoices for which the layout is adaptable in house. The posting can then either be made as a seperate process or built in to this software.

Billing requires a large volume of lines over about 50 invoices a month but these are all raised in a short cycle during the month.