customer database and invoice processing

I want someone to take my existing data input form and change it so that instead of it only entering customer details and sales info into a mysql database, it will also process the sales information and create an invoice page which can be printed. At the moment, keystrokes and mouse clicks are being wasted, entering the same data into two separate forms. The online MYSQL database may need some extra fields and/or a new table, too. For the invoice page, the sales info will need to be processed, so that prices minus VAT are displayed alongside the total price. I also want to have a print button next to the address text area on the input form, which will link to my address label printer software. Similar print buttons can go in other places, too. Likewise, I want to be able to call up invoice pages to print at any time, in case an invoice doesn't get printed at the time the operator enters customer and sales info into the database. Before my sales business got off the ground, I had time to tinker with php and mysql, teaching myself scripting as I went along, but now I'm way too busy to do this job by myself. Maybe there is a complete off-the-shelf solution I can use, scrapping my current system?