Change to AppCenter-like HumbleButton

[cassidyjames]

Instead of $10. $20, $30, Custom, do a "Pay $10" button w/a dropdown for custom

[danirabbit]

So few people choose anything other than the lowest button amount $10 or entering a custom $1, that we probably don't lose anything by dropping all other buttons completely tbh. I wonder if it would make sense to have a spin button type design

[danirabbit]

It looks like Checkout has an option for "customer sets price" when you create products there, so if we used that we would get this UI for free.

The only problem is that it doesn't allow selecting 0 so we'd need an alternative method for downloading for free