Easy Digital Downloads Clockwork Connect:
The Easy Digital Downloads Clockwork Connect extension allows you to integrate with ClockworkSMS and send instant notifications whenever purchases are made. Notifications can be sent as a summary or itemized list.
ClockworkSMS is a very user and pocket-friendly SMS API system. You can just use this plugin and all the order updates, notifications etc. can be sent directly to the customer’s mobile number. The customer is also happy that he gets a message on updates.
SMS are more read as compared to emails and so you should really try this system. You can surely check Clockwork’s rates directly on their pricing page.