Reading up on BoxCryptor it looks like it uses end-to-end encryption. So the data is encrypted before it leaves your computer. I believe the sign up is because it's a software subscription model, you have to pay annually, which I don't like. I'd rather just pay for the software outright, and buy upgrades in my own time.

Could you not add a layer of encryption to Dropbox, such as BoxCryptor (https://www.boxcryptor.com/)?