We received the following notice today:
Effective January 16, 2017 the Province of Ontario will no longer accept payments by AMERICAN EXPRESS CREDIT CARD.
The Province continues to accept payments by VISA CREDIT CARD and MASTERCARD CREDIT CARD when making a payment through the Ontario Parks reservation service. Payment by cheque or money order can also be made if you wish to make a reservation through the Ontario Parks call centre. Please ensure you have an applicable method of payment when making your 2017 Ontario Parks reservation through either our website or call centre.
Ontario Parks
Source: Boundary Waters Blogger