The service works by providing you with your own personal US shipping address. That address can then be used when making a purchase at ANY retailer in the US. When the package arrives at that address, it is automatically forwarded internationally to the final destination. For more details and information on how this works look at their site here.
UPDATE: If you were interested in this service, please take a look at the comments for this post. Sadly it was pointed out that this service does not allow shipping of nail polish, but there might be similar serviced that do...
Just as a disclaimer... I'm in no way associated with MyUS.com and have not personally used their service. I just thought it might interest those of you outside of the US who want to purchase items, but cannot get them shipped directly to you. For more information and shipping costs take a look at the MyUS.com website. Make your own decision if this service/additional cost is right for you. I'm in no way liable for any transactions made using this service (sorry, had to get that off my chest just in case :)).
Thanks for looking!