Here's how it's going to look like from the end user perspective:
1 hour before my PRO membership ends, if I am on a recurring plan Hummingbird will try to charge my credit card. If it succeeds everything is good. If it fails my subscription will run out and Hummingbird will try to charge my credit card again tomorrow, 3 days from now and 5 days from now. I will receive an email from Hummingbird if the charge is successful, and a different email every time charging my card fails.