the app for {company}, and when we spoke the only payment for our efforts was to "take a look at it" via a 15-minute video conference.

At that time you agreed, but you didn't' keep our call with no response from you.

{first_name}, reverse the roles and I'm sure you don't like your time / money wasted. 

If you'd like to try it, I'll send you a link to where you can download it.
If not, the best business practice I find and follow is in the Bible: "Let your yes be yes, and your no be no."