I discovered a bug before I even started playing.
Your upgrade screen does not handle exact change well. I started with 1,000 crystals, I bought Magnet 3 and Lives 2. That leaves me with precisely 300 crystals, and the first bullet upgrade costs... 300 crystals. But when I hover over the upgrade button, it gives me a red number, and it will not let me purchase it.
You're using a check for "Greater than zero" to determine if someone has sufficient crystals to buy something. You want to check for "Greater than or Equal to zero", or "Greater than Negative One."