After investigate this long hour ago, it seems like your current template styling conflict with Payplans 3.7.
I've applied some fix into your site :
1.
ROOT/components/com_payplans/xiframework/media/js/xi.core-min.js
Find this following code :
<div id="xiWindowContent" class="modal hide fade loading" tabindex="-1" role="dialog" aria-labelledby="myModalLabel" aria-hidden="true"></div>
Replace with
<div id="xiWindowContent" class="modal hide fade" tabindex="-1" role="dialog" aria-labelledby="myModalLabel" aria-hidden="true"></div>
2.
ROOT/components/com_payplans/media/css/payplans.css
find this css
div.modal {
position: fixed;
top: 10%;
left: 50%;
z-index: 1050;
width: 560px;
margin-left: -280px;
background-color: #ffffff;
border: 1px solid #999;
border: 1px solid rgba(0, 0, 0, 0.3);
*border: 1px solid #999;
-webkit-border-radius: 6px;
-moz-border-radius: 6px;
border-radius: 6px;
outline: none;
-webkit-box-shadow: 0 3px 7px rgba(0, 0, 0, 0.3);
-moz-box-shadow: 0 3px 7px rgba(0, 0, 0, 0.3);
box-shadow: 0 3px 7px rgba(0, 0, 0, 0.3);
-webkit-background-clip: padding-box;
-moz-background-clip: padding-box;
background-clip: padding-box;
}
Add the height attribute after width e.g.
height:400px;
3. Apply this following custom css into this file
body .modal-backdrop {
position: inherit;
}
JoomlaFolder/templates/responsive2/css/custom.css
Can you clear your browser cache and see how it goes?