Add a fee to the initial and recurring value when using Stripe including when a payment plan is selected.

mypmpro-change-price-plus-payment-plan-based-on-gateway.php

<?php
/**
 * This recipe will add a fee to the initial and recurring value when using Stripe
 * including when a payment plan is selected.
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
function my_pmpro_checkout_level( $level ) {
    if ( ! empty( $_REQUEST['gateway'] ) && $_REQUEST['gateway'] === 'stripe' ) {
        $level->initial_payment += 3;  // Adds fee to the initial payment
        $level->billing_amount += 3;   // Adds fee to the recurring payment

        // Check if a payment plan is selected
        if ( ! empty( $_REQUEST['payment_plan'] ) ) {
            $payment_plan_id = sanitize_text_field( $_REQUEST['payment_plan'] );

            // Adjust payment plan billing amount as well
            if ( isset( $level->payment_plans ) && is_array( $level->payment_plans ) ) {
                foreach ( $level->payment_plans as &$plan ) {
                    if ( $plan['id'] == $payment_plan_id ) {
                        $plan['initial_payment'] += 3;  // Adds fee to the plan's initial payment
                        $plan['billing_amount'] += 3;   // Adds fee to the plan's recurring payment
                    }
                }
            }
        }
    }
    return $level;
}
add_filter( 'pmpro_checkout_level', 'my_pmpro_checkout_level' );