This is mostly a framing issue. I am sure you are having "something" that is different from your competitors. The "Unique Selling Proposition" is getting increasingly replaced by the term "Value Proposition". And "Value Proposition" can be anything your company overdelivers - maybe your service is better, maybe your servers are more stable, maybe your user experience is better. That should be possible to argue a lot easier.
We believe we have a lot better customer service because I personally care a lot about our customers. That's actually the main reason we have decided to be more transparent with our pricing but from an investor point of view it's not something that we can prove or measure, am I right?
You certainly can do some post-customer care survey and collect personal testimonials of happy customers who praise how great your service was. But these better be really awesome, otherwise it looks too forced.
