If you're using fastmail you don't even have to worry about that limit. If your email is example@fastmail.com and you are registering with http://www.onlinestore.com/

Just use onlinestore@example.fastmail.com and fastmail will make sure it arrives in your example@fastmail.com inbox.

It's completely hassle free.

You'll still be uniquely identified. Just using fastmail itself is going to greatly narrow the search for your real identity.

Except with Fastmail you can use your own domain so it's not immediately obvious that you are using Fastmail.

Wow - been a Fastmail user for about a decade and have always used aliases (though I'm limited to just 10, I believe, based on my account type).

But this will come in handy!