magento2 checkout – How to Enable and Disable Payment Method in Magento 2

Theres a really handy event for disabling and enabling payment methods on the fly.

Add the following into your config.xml section

<payment_method_is_active>
    <observers>
        <preorder_payment>
            <class>your_module/observers</class>
            <method>invoiceOnly</method>
        </preorder_payment>
    </observers>
</payment_method_is_active>

Then create the following function in your observer.php

/**
 * Disable all other payments
 * @param Varien_Event_Observer $observer
 */
public function invoiceOnly(Varien_Event_Observer $observer) {

    $_result = $observer->getResult();
    $_methodInstance = $observer->getMethodInstance();

    if(BILLING_CONDITION_HERE) {
        if($_methodInstance->getCode() != 'METHOD_CODE') {
            $_result->isAvailable = false;
        } else {
            $_result->isAvailable = true;
        }
    }

}

You'll need to modify the BILLING_CONDITION_HERE and the METHOD_CODE (to the invoice payment method code)

This will automatically disable everything except METHOD_CODE if the billing condition is met.

One way to achieve this is in the file: app/design/frontend/base/default/template/checkout/onepage/payment/methods.phtml When looping over the selected methods add something like:

<?php if($_code == 'YOUR METHOD'): ?>
     <span class="no-display"><input id="p_method_<?php echo $_code ?>" value="<?php echo $_code ?>" type="radio" name="payment[method]" checked="checked" class="radio" /></span>
    <?php $oneMethod = $_code; ?>
<?php endif; ?>
    <label for="p_method_<?php echo $_code ?>"><?php echo $this->escapeHtml($this->getMethodTitle($_method)) ?> <?php echo $this->getMethodLabelAfterHtml($_method) ?></label>

Then add some JS at the bottom:

<script type="text/javascript">
//<![CDATA[
<?php if (is_string($oneMethod)): ?>
payment.switchMethod('<?php echo $oneMethod ?>');
    <?php endif; ?>
//]]>

In reality this is all that default magento template is doing to help the user select only a single option if one is set via the admin. However what I think might be happening in your case is that you have 2 methods available even though one may not be visible as a option. Adding this to the template will select that option by default and move to next step.