$50 to answer a question

I'm so tired and desperate, I'd love to pay for just an answer. I'm using D8 with Rules. Entire site is done, down to one last problem. I'm trying to create a reaction rule, with the event "After saving payment" or "after saving new order" either one. After the event, I want to calculate a new value in a field in the User's account. No matter what I do, I cannot get the custom field to appear in the data selector. I've tried many combinations of "entity has field" "entity is of type" "entity is of bundle" and "fetch entity by id". I'm lost, and simply need the correct combination of conditions and actions to get my custom field to appear in the data selector. It's a payment event, but the custom field is in the User entity, it's a custom decimal field I added to the user registration form. All too happy to pay for an answer. thanks!