I bought tools from two vendors that were both pitched as a one-time payment. They did not turn out the same.
The first vendor (Artistly, CloneVoice, TalkingPhotos, VideoExpress) has been one payment so far. I bought the bigger package, and in my own use the usage has been unlimited and updates have been included. I'm still testing VideoExpress.
The second vendor (ToonBee, ClipBee, InstaDoodle) went differently for me. I got a basic version. In my experience, selling what I made commercially needed a paid Pro upgrade. The credits were not unlimited, and some features were sold separately. Added up, it was not one payment.
These are the questions I'd ask now before any lifetime deal. Is the Pro or commercial license a separate purchase? Are any features sold on their own? If credits come with it, are they capped? That list comes from only two vendors, so it is probably incomplete.
How do you work out the real total before you pay? Has a lifetime deal ever cost you more than you expected?