Question: I have made a webapp for my android phone where I can check the sales at my store when I'm not there.

I would like to have notifications on my phone whenever a sale is made.

What is the best way of doing this? Any existing android apps that I can configure to check a php-script every 2 minutes or something?

Answer: 
Try using XMPP to send a message to yourself, which you can receive via gtalk on the phone. Alternatively, an email, SMS, etc.
Could you rig up an RSS feed and use a feed reader to notify you?