vasyakrn,
чем type=radio не устраивает?
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style>
.buy-param{
display: inline-block;
background: linear-gradient(rgb(255, 255, 255), rgb(206, 206, 206)) rgb(236, 236, 236);
padding: 2px 27px;
border-radius: 3px;
font: 12px Arial;
border-width: 1px;
border-style: solid;
border-color: #CCC #CCC #AAA;
}
</style>
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.11.0/jquery.min.js"></script>
<script>
$(function(){
var s = $(":checkbox");
s.change(function() {
s.not(this).filter('.'+this.className).prop({'checked' : false});
});
});
</script>
</head>
<body>
<div>
<label>
<input class="buy-chexbox" onchange="calculateTotalPacketCost()" type="checkbox" name="user_packet_groups[]" value="{$key}" />
<span class="buy-param">Купить</span>
</label>
</div>
<div>
<label>
<input class="buy-chexbox" onchange="calculateTotalPacketCost()" type="checkbox" name="user_packet_groups[]" value="{$key}" />
<span class="buy-param">Купить</span>
</label>
</div>
<div>
<label>
<input class="buy-chexbox" onchange="calculateTotalPacketCost()" type="checkbox" name="user_packet_groups[]" value="{$key}" />
<span class="buy-param">Купить</span>
</label>
</div>
</body>
</html>