Define payment plans by mapping a level to discount codes representing payment plan options

my_pmpro_payment_plan_checkout_boxes.php

<?php

/**
  * Create payment plans by mapping a level to discount codes representing payment plan options.
  * Useful for offering multiple pricing structures for membership (i.e. Monthly, Annually)
  *
  * Add this code below to your PMPro Customizations Plugin - https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
  */

global $pmpro_payment_plans;

// Define the payment plans. '3' => array( 11, 12, 13 ) means that membership level with the ID of 3 can be paid using discount codes with the ID 11, 12, and 13
$pmpro_payment_plans = array(
	'3' => array( 11, 12, 13 ),
);

// Show the "Select a Payment Plan" box with options at checkout.
function my_pmpro_payment_plan_checkout_boxes() {
	global $pmpro_payment_plans, $wpdb;
	if ( empty( $_REQUEST['level'] ) || empty( $_REQUEST['discount_code'] ) ) {
		return;
	}

	$level_id         = $_REQUEST['level'];
	$discount_code    = $_REQUEST['discount_code'];
	$discount_code_id = $wpdb->get_var( $wpdb->prepare( "SELECT id FROM $wpdb->pmpro_discount_codes WHERE code=%s LIMIT 1", $discount_code ) );

	// Make sure passed discount code is valid.
	if ( ! pmpro_checkDiscountCode( $discount_code, $level_id ) ) {
		return;
	}
	// Group doesn't have a payment plan or using a different discount code? return.
	if ( empty( $pmpro_payment_plans ) || ! isset( $pmpro_payment_plans[ $level_id ] ) ) {
		return;
	}

	// Get payment options.
	$payment_options = $pmpro_payment_plans[ $level_id ];
	if ( ! is_array( $payment_options ) ) {
		$payment_options = array( $payment_options );
	}

	// Make sure discount code is a payment option.
	if ( ! in_array( $discount_code_id, $payment_options ) ) {
		return;
	}

	// Create payment plan box.
	?>
	<div id="pmpro_level_options" class="pmpro_checkout">
		<h3><span class="pmpro_checkout-h3-name"><?php esc_attr_e( 'Select a payment plan.', 'paid-memberships-pro' ); ?></span></h3>
		<div class="pmpro_checkout-fields">
			<div class="pmpro_checkout-field pmpro_checkout-field-radio">
				<?php
				foreach ( $payment_options as $payment_option ) {
					// Make sure discount code is valid.
					if ( ! pmpro_checkDiscountCode( $wpdb->get_var( $wpdb->prepare( "SELECT code FROM $wpdb->pmpro_discount_codes WHERE id=%d LIMIT 1", $payment_option ) ), $level_id ) ) {
						continue;
					}

					// Get discount code infomation.
					$sql_query = "SELECT * FROM $wpdb->pmpro_discount_codes_levels WHERE code_id = '" . $payment_option . "' AND level_id = '" . (int) $level_id . "' LIMIT 1";
					$payment_plan_level = $wpdb->get_row( $sql_query );
					$sql_query = "SELECT code FROM $wpdb->pmpro_discount_codes WHERE id = '" . $payment_option . "'  LIMIT 1";
					$code_name = $wpdb->get_row( $sql_query )->code;

					// Apply filters.
					$payment_plan_level = apply_filters( 'pmpro_checkout_level', $payment_plan_level );
					?>
					<div class="pmpro_checkout-field-radio-item">
						<input type="radio" id="pmpro_code_<?php echo $payment_option; ?>" class="my_pmpro_payment_plan_option" name="my_pmpro_payment_plan_option" value="<?php echo $code_name; ?>" <?php checked( $payment_option == $discount_code_id ); ?> >
						<label class="pmpro_label-inline" for="pmpro_code_<?php echo $payment_option; ?>" /><?php echo pmpro_getLevelCost( $payment_plan_level, false, true );?></label>
					</div>
					<?php
				}
				?>
			</div>
		</div>
	</div>
	<script>
	jQuery(document).ready(function() {			

		// Prevent changing discount code manually
		jQuery('#other_discount_code_p').wrap("<div id='other_discount_code_p_hidden' style='display: none;'></div>");

		// Hide 'discount code updated' message in level cost text on first page load.
		if( jQuery ( "#pmpro_level_cost p" ).length > 1) {
			jQuery('#pmpro_level_cost p').first().hide();
		}

		// Update discount code when different payment plan is chosen
		jQuery('.my_pmpro_payment_plan_option').click(function() {
			// Get chosen payment plan
			code = jQuery('.my_pmpro_payment_plan_option:checked').val();
			// Set corresponding discount code
			jQuery( '#other_discount_code' ).val( code );
			// Update discount code
			jQuery('#other_discount_code_button').click();
		});

		// Save original checkout message
		pmpro_original_message = jQuery('#pmpro_message')[0].outerHTML;
		// After any AJAX call...
		jQuery( document ).ajaxComplete(function() {
			// If the current discount code was updated...
			if( jQuery ( "#pmpro_level_cost p" ).length > 1) {
				// Hide 'discount code updated' message in level cost text
				jQuery('#pmpro_level_cost p').first().hide();
				// Restore the original checkout message
				jQuery('#pmpro_message')[0].outerHTML = pmpro_original_message;
			}
		});
	});
	</script>
	<?php
}
add_action( 'pmpro_checkout_boxes', 'my_pmpro_payment_plan_checkout_boxes' );

function my_pmpro_automatically_give_payment_plan() {
	global $pmpro_payment_plans, $wpdb;

	// Make sure that there is a level and not already a discount code.
	if ( empty( $_REQUEST['level'] ) || ! empty( $_REQUEST['discount_code'] ) ) {
		return;
	}
	$level_id = $_REQUEST['level'];

	// Group doesn't have a payment plan? return.
	if ( empty( $pmpro_payment_plans ) || ! isset( $pmpro_payment_plans[ $level_id ] ) ) {
		return;
	}

	// Get payment plan.
	$payment_plans = $pmpro_payment_plans[ $level_id ];
	if ( ! is_array( $payment_plans ) ) {
		$payment_plans = array( $payment_plans );
	}

	// Give starting payment plan (aka a discount code).
	if ( ! empty( $payment_plans ) ) {
		$code = $wpdb->get_var( $wpdb->prepare( "SELECT code FROM $wpdb->pmpro_discount_codes WHERE id=%d LIMIT 1", $payment_plans[0] ) );
		if ( pmpro_checkDiscountCode( $code, $level_id ) ) {
			$_REQUEST['discount_code'] = $code;
		}
	}
}
add_action( 'init', 'my_pmpro_automatically_give_payment_plan' );

A version with a UI to show a custom name for each plan instead of the level cost is at https://gist.github.com/gausam/b9bbe5f81be51b3b5a8cc5e9dc241be6

This recipe is included in the blog post on "Allow members to select a Payment Plan for membership at checkout (discount code method)" at Paid Memberships Pro here: https://www.paidmembershipspro.com/add-a-payment-plan-to-a-pmpro-checkout-page/

If you change line 117 to use pmpro_checkout_after_level_cost the options will show up closer to the top of the form.