[Plugin: WordPress Simple Paypal Shopping Cart] Error in the connection between Paypal and the shop

I use the E-store WordPress theme on my site http://www.tygeriet.se . Now it has accured a problem when customers try to order part of meter. For example 1.2 meters. They get an error message from Paypal that says “You have entered an invalid quantity value. A quantity value must be an integer greater than or equal to one.” Why is that so? In the beginning there was no problem at all. Do you have any suggestions what to do?

http://wordpress.org/extend/plugins/wordpress-simple-paypal-shopping-cart/