A friend of mine who I made a website for using VSD would like to sell vouchers (he owns a fishing lake!), on his website. I have done similar with a lady who wanted to sell her clipart. I created a shop, and a thumbnail image of the product for sale with a watermark on it. When the customer checked out using the paypal button, he/she was immediately redirected to a new page holding the non-watermarked clipart. Not totally secure, but no problems have been reported. This new idea is for vouchers, of course, once a customer has checked out, the voucher can be easily duplicated, so what I am looking for is a way of adding a code generator to the page, or maybe even a page counter? Any ideas?

A code generator can be as simple as the date of purchase if you include seconds it should be unique.
A form can output to a MySQL database which usually has a primary field . This can be extracted as can the stored purchase date. each is unique and can be used on the voucher. It will require additional manipulation and someone with php knowledge