“Add Tip” does not always work

Hi,

I have the plugin set up on the checkout page only and have tested it using safari, chrome and firefox

The add tip button does not work the first time it is pressed.

Sometimes it works on the second time, sometimes it takes 4-5 times for it to work.

Any help you can offer it greatly appreciated.

thanks,

Brent

The page I need help with: [log in to see the link]