Simple PayPal Shopping Cart adding a text box

I am trying to edit a short code so that one of the variables produces a text box. I am creating an area on my website where members of our club can purchase club polo shirts in a variety of sizes and colours. I also want people to be able to add that they want a name on the shirt with an area for them to type the name in for each product they order.
So far I have the following shortcode [wp_cart_button name=”Junior Polo Shirt” price=”6.00″ var1=” Size|Small|Medium|Large|XL|XXL” var2=”Colour|Bottle Green|White”] but I am struggling to adda text area.
The plugin I’m using is Simple PayPal Shopping Cart and for the life of me I can’t see anywhere in the documentation on how to do it.

If anybody can help I would appreciate it.

Regards

Jon McLaughlin