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

my_pmpro_additional_categories.php

/*
  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");

This recipe is included in the blog post on "Sell Access to Post Categories “à la carte” Using Price-Adjusting Register Helper Fields" at Paid Memberships Pro here: https://www.paidmembershipspro.com/sell-access-to-helper-fields/