Ruby on Rails

So you are going to have one text field that gathers membercode?  And associated with the membercode are going to be a bunch of bottons, each that will provide some controller action on membercode.   Will the action always be  def create?
You have your recover_letter_rpts_path(:membercode).  I am assuming that this path as defined in your router.rb and leads to controller action 'create'.    Run rake routes and show us the result, or copy/paste your route.rb here. 
Don't understand: "And there is no form to contain these fields."  Plural fields.  These fields?:
<%= label_tag :"Enter member code" %>
<%= text_field_tag :membercode%>
<%= button_to "Death/Retired Recovery Letter Printing", recovery_letter_rpts_path(membercode: membercode) %>
Are you saying that this code is not contained in a form...?    Otherwise, show us the form tag.
Is membercode being returned in your params?    Have you been able to capture that param in your controller?
Pending your answers to the above, a possible solution is this...
If all buttons lead to def create...
Go to your router and modify  recover_letter_rpts_path(:membercode).   Add a new parameter, let's call it :process.   Then for the above button, you could rewrite it as..
<%= button_to "Death/Retired Recovery Letter Printing", recovery_letter_rpts_path(membercode: membercode, process: 'verify_existence') %>   I'm not sure about the best syntax for adding a second parameter
In your controller
def create
    membercode = params[:membercode]   ## should verify existence (blank?) of params[:membercode]
    membercode_process = params[:process]   ## should verify existence (blank?) of params[:process]
   if  membercode_process == 'verify_existence'
       member_exist = Member.find_by_member_code(membercode)   ##  pretty sure that 'exist' is a deprecated method in Rails 2.  I'd use something like member_exist
        respond_to do |format|
          if member_exist.blank??       ## blank? is more thorough, it tests for object.nil? || object.empty?
              ##  puts is usually done for debugging purposes, do you want to provide this information to your user?  If so, you may want to put this content into a partial -- depending on how elaborate your messages become -- which will be rendered in recoveries_path.
              ###  Or you could store it in flash, which will be presented in recoveries_path
                    puts "******************************************"
             puts "This is the member code #{membercode}"  # done
             puts "******************************************"
        redirect_to(recoveries_path)
      else
        puts "This is the NO member code #{membercode}"   ## Same as above
        redirect_to(recoveries_path)
      end
elsif     membercode_process == 'some_other_process'
etc.....
end
end   
end
If all buttons don't lead to def create... then create a new route path for each.... But I think that folks here will have better suggestions in that case.