Yes. You may cancel at any time. Just log into your account and go to the Account Settings > Billing page in your account.
Accountable2You is based in the United States and all subscription charges are made in US Dollars. If your card supports foreign transactions you may pay using this method, and your bank will apply the appropriate exchange rate to the charge. Contact your bank for more details.
You may use a debit or credit card. Both are accepted and work equally well.
Accountable2You monitors the activity of the person signed into a device. Since most smartphones, tablets, and computers only have one user profile, it will require one active install to monitor that device. However, shared devices may have more than one user profile. For instance, if a family computer has a user profile for dad, one for mom, and another for the kids, it will require one install to monitor each profile, resulting in three active installs. Why limit the number of active installs? We've answered that here