Why is 50 USD the minimal contribution? Is there an address where I can send any amount?
The $50 minimum is due to a limitation in the size of the final transaction.
Have you used Flipstarter before?
I guess you can directly donate to Mark (`bitcoincash:qqy9myvyt7qffgye5a2mn2vn8ry95qm6asy40ptgx2`), but it won't be included in the Flipstarter campaign.