I have a Kindle and an iPad. My Kindle is connected to my Amazon account. My iPad is connected to my Apple account and to my Amazon account, because I use the Kindle App on my iPad. I also download my digital library books through Amazon to my Kindle App.
I totally trust these two companies and have no problem with giving them a credit card number. We have never had a problem with either company, in twenty years. In fact, once I bought a lousy app from Apple, and they refunded my money to me.
However, we purchased a Roku last winter to use for the TV in our new room addition. They wanted a credit card number. Somehow, we worked around it, but I'm sure it will come up again. I will just give them a number of a credit card that we never use. That way, I'll easily know if something is charged.
"I've been here since October 2006. Wow!"