Add "Purchase Additional Access" box to Membership Checkout for a la carte category purchase.

my-pmpro-additional-categories.php

<?php // Do not include this line in your Customizations plugin
/**
 * Require specific category user meta to view posts in designated categories.
 * Custom price-adjusting fields are added via PMPro Register Helper add on (required).
 * Add all of this code to your active theme's functions.php or a custom plugin.
 */
add_filter( 'pmpro_has_membership_access_filter', 'my_pmpro_additional_categories', 10, 4 );
function my_pmpro_additional_categories( $hasaccess, $thepost, $theuser, $post_membership_levels ) {
	global $wpdb;

	// if PMPro says false already, return false
	if ( ! $hasaccess ) {
		return false;
	}

	// if the post is in the category and the user has not purchased the category, then block
	if ( has_category( 'cooking', $thepost ) ) {
		$cooking = get_user_meta( $theuser->ID, 'cooking', true );
		if ( empty( $cooking ) ) {
			$hasaccess = false;
		}
	}
	if ( has_category( 'baking', $thepost ) ) {
		$baking = get_user_meta( $theuser->ID, 'baking', true );
		if ( empty( $baking ) ) {
			$hasaccess = false;
		}
	}
	if ( has_category( 'crafts', $thepost ) ) {
		$crafts = get_user_meta( $theuser->ID, 'crafts', true );
		if ( empty( $crafts ) ) {
			$hasaccess = false;
		}
	}

	return $hasaccess;
}

// add fields
function my_pmprorh_init() {
	// don't break if Register Helper is not loaded
	if ( ! function_exists( 'pmprorh_add_registration_field' ) ) {
		return false;
	}

	$fields[] = new PMProRH_Field(
		'cooking', // input name, will also be used as meta key
		'checkbox', // type of field
		array(
			'text' => 'Cooking Category ($10)',
			'memberslistcsv' => true,
			'profile' => 'admin_only',
			'showmainlabel' => false,
		)
	);
	$fields[] = new PMProRH_Field(
		'baking', // input name, will also be used as meta key
		'checkbox', // type of field
		array(
			'text' => 'Baking Category ($10)',
			'memberslistcsv' => true,
			'profile' => 'admin_only',
			'showmainlabel' => false,
		)
	);
	$fields[] = new PMProRH_Field(
		'crafts', // input name, will also be used as meta key
		'checkbox', // type of field
		array(
			'text' => 'Crafts Category ($10)',
			'memberslistcsv' => true,
			'profile' => 'admin_only',
			'showmainlabel' => false,
		)
	);
	// add the fields into a new checkout_boxes are of the checkout page
	pmprorh_add_checkout_box( 'additional_categories', 'Purchase Additional Access' );
	foreach ( $fields as $field ) {
		pmprorh_add_registration_field(
			'additional_categories', // location on checkout page
			$field // PMProRH_Field object
		);
	}
	// that's it. see the PMPro Register Helper readme for more information and examples.
}
add_action( 'init', 'my_pmprorh_init' );

/*
  If a user checked options, then adjust the price.
*/
function my_pmpro_checkout_level( $level ) {
	if ( ! empty( $_REQUEST['cooking'] ) ) {
		$level->initial_payment = $level->initial_payment + 10;
		$level->billing_amount = $level->billing_amount + 10;
	}
	if ( ! empty( $_REQUEST['baking'] ) ) {
		$level->initial_payment = $level->initial_payment + 10;
		$level->billing_amount = $level->billing_amount + 10;
	}
	if ( ! empty( $_REQUEST['crafts'] ) ) {
		$level->initial_payment = $level->initial_payment + 10;
		$level->billing_amount = $level->billing_amount + 10;
	}

	return $level;
}
add_filter( 'pmpro_checkout_level', 'my_pmpro_checkout_level' );