The PMPro code snippet in https://www.paidmembershipspro.com/multiple-membership-levels-per-user-pmpro-workarounds/ does not work as-is. It is a very simple premise, only checking for one option. Recurring payments do not work the way it was originally written either. This does the same thing using a checkbox and only allowing for a one-time pay…

my_pmpro_adjustable_level_cost_v1.1.php

<?php
function my_pmpro_adjustable_level_cost($level)
{
    if(isset($_REQUEST['field_name']))
    {
        $level->initial_payment = $level->initial_payment + 100; //specify the cost of the selected option
        // You cannot update recurring payments with this code as-is, see comment below
    }

    return $level;
}
add_filter("pmpro_checkout_level", "my_pmpro_adjustable_level_cost");
?>

THIS VERSION OF THE PLUGIN IS NOT MUCH DIFFERENT FROM THE "ADDON PACKAGES" ADDON. IF THIS IS ALL YOU NEED TO DO, THAT ADDON MIGHT BE A BETTER OPTION, BUT FOR MORE FUNCTIONALITY SEE THE MORE ADVANCED VERSIONS OF THIS CODE SNIPPET BELOW.

To implement this functionality, navigate to the Memberships > Settings > User Fields page, create a new field group and call it something like, “Add X to Your Plan.” Select “Yes” for “Show fields at Checkout,” and choose your Levels in the “Restrict Fields for Membership Levels” dropdown. Next, add a field of type “checkbox,” name it something in the Label field, make note of the generated Name field and input that as the field_name in the code. Be sure to click “Save All Changes” at the top of the User Fields settings page in Paid Memberships Pro. Adjust the number value of the additional amount (currently set to 100).

Need more functionality? Check these out:
Version 1.1 of the code only allows for a single checkbox and a one-time payment.
Version 1.2 of the code that only allows for a single checkbox and works for one-time payments and recurring plans.
Version 1.3 of the code that only allows for a single checkbox for one time payments and recurring plans and also adjusts the cost of the option based on if it's a monthly or annual plan.
Version 1.4 of the code that adds more options and allows for recurring plans.
Version 1.5 of the code to add more options, allow for recurring plans, and adjusts the cost based on if it's a monthly or annual plan.

There are a couple of housekeeping items to ensure you are displaying accurate information to your users. In Memberships > Settings > Email Templates, select the email template “Checkout - Paid,” and “Checkout - Paid (admin)” and remove the following line: <p>Membership Fee: !!membership_cost!!</p> That line not only displays the initial cost of the membership plan and not the updated membership plan with the options, but the way that it is phrased: “Membership Fee: The price for membership is $x now, and $x every Month” may confuse users who did not purchase a renewable plan (I have contacted PMPro about this).

Finally, if you have the Auto-Renewal Checkbox addon, the checkbox label includes a code-generated calculation of the plan cost. This is also no longer accurate once our custom code is implemented. To update the calculation based on the selected options would require Ajax to handle the checkbox toggle event and a server-side request to calculate and return the calculation, as well as significant modifications to our custom code. That is beyond the scope of this tutorial. Instead, let’s just replace the label of the checkbox with the static text, “Renew your plan” using jquery. Place this code in your child theme’s functions.php file:

function custom_auto_renewal_checkbox_label() {
    if ( is_page( 'membership-checkout' ) ) {
        ?>
        <style>
            /* Hide the original auto-renewal checkbox label initially */
            label[for="autorenew"].pmpro_clickable {
                visibility: hidden;
            }
        </style>
        <script>
            jQuery(document).ready(function($) {
                    // Show the updated auto-renewal checkbox label
                    $('label[for="autorenew"].pmpro_clickable').css('visibility', 'visible');
                    $('label[for="autorenew"].pmpro_clickable').html('Automatically renew your plan.');
            });
        </script>
        <?php
    }
}
add_action('wp_footer', 'custom_auto_renewal_checkbox_label');