I figured it out!
I replaced
<button type="button" title="<?php echo $this->__('Select') ?>" class="button btn-cart" onclick="setLocation('<?php echo $this->getAddToCartUrl($_product) ?>')"><span><span><?php echo $this->__('Select') ?></span></span></button>
with
<button class="button related" type="button" onclick="location.href='<?php echo $this->getAddToCartUrl($_link) ?>'"><span><?php echo $this->__('Add to Cart') ?></span></button>
And put this at the bottom:
<script type="text/javascript">
//<![CDATA[
var productAddToCartForm = new VarienForm('product_addtocart_form');
productAddToCartForm.submit = function(button, url) {
if (this.validator.validate()) {
var form = this.form;
var oldUrl = form.action;
if (url) {
form.action = url;
}
var e = null;
try {
this.form.submit();
} catch (e) {
}
this.form.action = oldUrl;
if (e) {
throw e;
}
if (button && button != 'undefined') {
button.disabled = true;
}
}
}.bind(productAddToCartForm);
productAddToCartForm.submitLight = function(button, url){
if(this.validator) {
var nv = Validation.methods;
delete Validation.methods['required-entry'];
delete Validation.methods['validate-one-required'];
delete Validation.methods['validate-one-required-by-name'];
// Remove custom datetime validators
for (var methodName in Validation.methods) {
if (methodName.match(/^validate-datetime-.*/i)) {
delete Validation.methods[methodName];
}
}
if (this.validator.validate()) {
if (url) {
this.form.action = url;
}
this.form.submit();
}
Object.extend(Validation.methods, nv);
}
}.bind(productAddToCartForm);
//]]>
</script>
</div>