functions.php

<?php 

// this goes in your theme functions.php file

function my_custom_rest_dispatch_request($continue, $request, $route, $handler)
{
  if (function_exists('is_pos') && is_pos() && $route == 'wc/v3/products') {
    $request->set_param('per_page', -1);
    $request->set_param('stock_status', 'instock');
  }
  
  return $continue;
}

add_filter('rest_dispatch_request', 'my_custom_rest_dispatch_request');