Is it free to text a USA number if I have the Canada only plan?

If you send texts outside of your coverage zone (Canada only in your case), it will cost extra to send texts. Receiving text is free but sending will cost you.

Hi Peter,

Fizz is prepaid. So if a feature isn't included in your plan, you haven't bought add-on and you haven't added money to your wallet, then you cannot use that feature. And you will not be billed by surprise for it.

As long as you're physically within your coverage area (in your case within Canada), you can text to any number (including Canada, USA and International numbers).

And for incoming calls, you can receive all calls as long as you're physically within Canada (make sure you're connected to Canadian network if you're located close to USA border).
As for your outgoing calls, your plan includes calls to Canadian numbers.

And it does not matter to you where that someone is located. Whether someone receives your text/calls depends entirely whether their plan includes it.