The interface for Authorize.Net is a bit complicated. Also, opening a merchant account requires multiple parties, while other services such as PayStand are a one-stop shop. One of our previous web developers recommended Authorize.Net, but I wouldn't recommend it.
Dropbox is easy to use and gives you the ability to share resources that may be too large to submit via email. The only con is that, after a certain capacity, it requires you to subscribe and pay a monthly fee.