Don't expect customers to write you a blank cheque
I considered purchasing Mail Pro, then saw this:
"The amount charged may change. If you do not cancel the Service, you hereby consent that the payment method or, if your designated payment method fails, any other payment method that you have on file will automatically be charged the then-current fee for the Service at the commencement of each renewal period. Details of the current fees will be posted at https://payments.mail.yahoo.com."
Outrageous to think you can sell a service at a given price then start charging more than agreed to without even informing customers, as if anyone's going to be checking some obscure URL every day to monitor for changes. Is this even legal?
