is replying to an sms a reliable way of authenticating a response

I have a web app which will relay messages from my desktop applications (which is a workflow application for my client).

A typical message would be - 'you have booked your pc in for a service and we have agreed a fee for this work. But, we found a RAM stick needs to be replaced and we need your permission to do this for this new price. Please reply to this message with a 'y' or 'n' (yes or no).

This would be sent from my web server app to the Users registered mobile.

The mobile User would then reply 'y' or 'n'.

But is this a reliable way of doing things? What if the user gets confused by the message? What if they accidentally sent a 'y' instead of a 'n'.

Should I rely on this? Should I send another SMS to the mobile user for confirmation?

Would it be better to have a link in the SMS that directs them to the mobile browser where they have to enter a code and tick a box and then submit these details back to my server.

First time I have done anything like this and normally I try to educate myself via Googling but have not found any thing out there.