I used Evisa Express to buy the ESTA USA. From what I've read, it is a document essential to travel to America. If you don't have it, you will be required to go back home. I applied at their webiste and I must admit the application form was clear, but I have been waiting WEEKS for approval. Luckily I got it on time. Of course I contacted the support team and they said they did not issue my visa sooner because there was a long time between the application and departure date. Still, I'd be less worried if I knew it sooner... Good service and helpful staff but they shouldn't make you wait this long!