Welcome to the forum, @anajancikovic! Let’s have a look into this.
Are the charges you’re receiving direct Dropbox ones, or via iTunes/Google Play? What do you see when opening your bank statement?
From what you mention, there’s also a chance you’re being billed for a Dropbox account that’s different to the one you’re currently logged in to (and which is also linked to a different email address).
If you’re being charged directly from Dropbox, you can also visit this page, and insert your 12-digit transaction ID there to see the email address tied to the charges.
Let me know, and we'll take it from there.