Do you want to build this app because it's fun, do you want to invest in startups, or do you want to contract for cash?
What it sounds like you have is equity in the school of hard knocks.
The question you need to be asking yourself is: If you had $7500 in cash would the number 1 thing on your mind be investing in this company? If the answer is no, then don't convert your note. You can also neglect to convert your note and take cash. It sounds like you want cash up front.
The company now has $150K in the till, if they can't figure out how to get a guy who did 2X what they asked for $7500 then it's not a company you want to be an investor in. Go for a free consult with a lawyer and he'll explain the options your client is not providing you with.
Lessons learned:
Don't accept payment in convertible notes, accept cash up front.
Don't do 2X the work you're being paid for, talk to the client and get paid 2X.