I added the custom text fields First Name and Last Name to the User account - /admin/config/people/manage.
Next I created a Rule that sends an email. In the email I use replacement patterns for First and Last name. When the email is sent instead of displaying the First and Last name it just says "Array Array". Recent log messages include this notice - Notice: Array to string conversion in RulesTokenEvaluator->evaluate() (line 224 of .../modules/rules/modules/system.eval.inc). Am I doing something wrong or is this a bug?
Recent comments
For such cases I like EVA https://backdropcms.org/project/eva
Display field using content of another field
If you don't want/need a View, I think Computed Field would be an excellent option for this. BTW, you can use token replacement (by using the function token_replace()) in the computation of the field...
Display field using content of another field
Views would be my first port of call, but I guess you've already thought of that. I haven't used it but you could also look at Views Field Formatter
Display field using content of another field
What about Ubercart, Rules and Membership Entity? (theoretically, haven't tried it myself)
Suggest for - Backdrop Commerce
Just edit .htaccess in /files folder remove "-ExecCGI -Includes" in string "Options -Indexes -ExecCGI -Includes"
CSS and JS aggregation not working