Invoice.rb:
has_many :service_types, through: :invoice_service_type,

Invoice form:
  <div class="col-md-12"  id="ServiceType">
    <strong>Serviços:</strong>
     <%= f.simple_fields_for :invoice_service_types do |invoice_service_type| %><br>
      <%= render 'invoices/invoice_service_type_fields', f: invoice_service_type %>
     <% end %>
    </div>
    <br/><br/><%= link_to_add_association ' Novo Serviço', f, :invoice_service_types, class: 'fa fa-plus btn btn-success' %>
  </div>

Invoice Service Types, with fix name of service types, but different values for each Service Type and Invoice.
invoice_service_type_fields form:
<div class="nested-fields form-inline">
	<div class="col-xs-12 col-lg-12">
		<%= f.association :service_type, collection: ServiceType.all.each.map {|v, k|[v.name, v.id]}, prompt: 'Escolha o Serviço', label: false %>
		<%= f.input :value_charged, class: "form-control", placeholder: "Valor cobrado",  label: false %>
		<%= link_to_remove_association "", f, class:"fa fa-trash-o h3", style:'font-size: 20px;' %>
	</div>
</div>

InvoiceServiceType.rb
 belongs_to :invoice
 belongs_to :service_type

 Schema for InvoiceServiceType: ( this what take me a lot of time, to discover how to have diferent values for same kind os service type )
 create_table "invoice_service_types", force: :cascade do |t|
    t.float   "value_charged"
    t.integer "invoice_id"
    t.integer "service_type_id"
  end