My iPhone is displaying a message about paying a fee to activate it

That's a scam.

If it's coming from some dedicated app (rather than from, say, a Web page or an e-mail), I would be very wary of the app that presented it to you. If I, personally, saw an app try something like this on me, that would be a good reason to report that app to Apple after first deleting it off my phone!

Federal Trade Commission – Avoiding and Reporting Gift Card Scams

"Only scammers will tell you to buy a gift card, like a Google Play or Apple Card, and give them the numbers off the back of the card. No matter what they say, that’s a scam. No real business or government agency will ever tell you to buy a gift card to pay them."

Mac Jim ID wrote:

Second screenshot is fake. Time not displayed correctly and Visa obviously pasted in there.

Also, iPhone spelled with the capitalization "Iphone" – which Apple would never do unless they were being really careless. And "usd" in lower case when the convention is normally to write currency abbreviations in upper case ("USD").

What needs to be "activated" is a prison cell for the criminal who sent the message …

Cute. Here’s the exact same background behind the iPhone, and with very slightly different text, and sharing some of the same typos:

https://www.pinterest.com/pin/1024357877732299054/

Looks like some photoshopping happened here too, based on the text alignment errors, and the font usage.

For the font details, look at the “a” in the apparently-added “Visa” and the “a” used in the rest of the text.

Anyway. It’s not reality, as Apple doesn't charge for activations.

Not sure exactly how those screens were generated, but it’s def a scam.

There is no charge associated w/ activating an iPhone.

Second screenshot is fake. Time not displayed correctly and Visa obviously pasted in there.