All I want to do is make sure my yahoo mail is somehow saved in iCloud without it costing me more $ than yahoo is already charging me

Well, you could do something like take screenshots of your mail and store the screenshots using iCloud Photos.

But the bottom line is you don’t get something for nothing.

Of course, you can avoid this for future email by using iCloud mail instead of Yahoo.

Sounds good.

I am not personally familiar with Yahoo mail’s intricacies, but there is very likely a “Forward” function available so you can rely on iCloud mail to receive future mail from both iCloud and Yahoo.