[Plugin: WP Paypal Shortcodes] Possible to Add Blank Fields?

I would like to use this plugin and have two blank fields display on the page above the Pay button – Invoice # and Amount.

This way clients can enter their Invoive number and the amount owed to pay using paypal.

Is this even possible to add blank fields on the page?

Thanks!!

http://wordpress.org/extend/plugins/wp-paypal-shortcodes/