To add compatibility with T3 framework used from Joomlart make this change:
in file /plugins/system/jsn_system/jsn_system.php at line 266 replace this:

if($(this).children(".control-group.hide").length==$(this).children(".control-group").length){

with this

if($(this).children(".control-group.hide,.form-group.hide").length==$(this).children(".control-group,.form-group").length){

we will add this in next release

Also to keep a clean template you can add these CSS rule (maybe into Extensions->Template Manager->Ja Bookshop->Custom Code->Before HEAD):

<style>

.registration{border:none}

#profile_tabs{margin-bottom:20px;}

.registration form#member-registration fieldset legend + div.form-group{width:44%}

@media (max-width: 767px){

.registration form#member-registration fieldset legend + div.form-group {

width: auto;}}

.registration form#member-registration fieldset div.form-group{margin-bottom:10px;}

</style>