Hello there @Seller_bPqNLG6VShvB5

When you sign up for a selling plan you are automatically enrolled in the the North America Unified account which allows you ability to sell in US, Canada and Mexico.

Here is a 2 minute overview of the North America Unified Account should you want to review.

Additionally, this page has a lot of great resources including FAQ's on the account. Two of which are the below:

What if I don't want to sell on more than one store?

If you have been upgraded to a Unified Account and do not want to sell on another store, keep the store switcher on the country on which you originally registered. Your products will only list on that store.

What fees will I pay if I sell something using my Unified Account?

For all per-item transaction fees, you will pay the fees applicable to the store in which the item was sold. Your monthly Professional selling plan subscription fee will be allocated to each North American store in which you have active listings on your monthly billing day, and will be limited to USD 39.99 or its equivalent in total. For more information about Professional selling plan fees, go to Selling on Amazon Fee Schedule.

This post was also made by a fellow community manager that has additional information.

CR_Amazon