<div class="card-body">
  <div class="row mb-0">
    <div class="form-group col-6 mb-3">
      <%= select_tag "#{param_prefix}[preferred_operator_min]", options_for_select(Spree::Promotion::Rules::ItemTotal::OPERATORS_MIN.map{|o| [Spree.t("item_total_rule.operators.#{o}"),o]}, promotion_rule.preferred_operator_min), { class: 'select2 select_item_total mb-3' } %>
      <%= select_tag "#{param_prefix}[preferred_operator_max]", options_for_select(Spree::Promotion::Rules::ItemTotal::OPERATORS_MAX.map{|o| [Spree.t("item_total_rule.operators.#{o}"),o]}, promotion_rule.preferred_operator_max), { class: 'select2 select_item_total' } %>
    </div>
    <div class="form-group col-6 mb-3">
      <%= text_field_tag "#{param_prefix}[preferred_amount_min]", promotion_rule.preferred_amount_min, class: 'form-control mb-3' %>
      <%= text_field_tag "#{param_prefix}[preferred_amount_max]", promotion_rule.preferred_amount_max, class: 'form-control' %>
    </div>
  </div>
</div>

<div class="card-body">
  <div class="row mb-0">
    <div class="form-group col-6 mb-3">
      <%= select_tag "#{param_prefix}[preferred_operator_min]", options_for_select(Spree::Promotion::Rules::ItemTotal::OPERATORS_MIN.map{|o| [Spree.t("item_total_rule.operators.#{o}"),o]}, promotion_rule.preferred_operator_min), { class: 'custom-select select_item_total mb-3' } %>
      <%= select_tag "#{param_prefix}[preferred_operator_max]", options_for_select(Spree::Promotion::Rules::ItemTotal::OPERATORS_MAX.map{|o| [Spree.t("item_total_rule.operators.#{o}"),o]}, promotion_rule.preferred_operator_max), { class: 'custom-select select_item_total' } %>
    </div>
    <div class="form-group col-6 mb-3">
      <%= text_field_tag "#{param_prefix}[preferred_amount_min]", promotion_rule.preferred_amount_min, class: 'form-control mb-3' %>
      <%= text_field_tag "#{param_prefix}[preferred_amount_max]", promotion_rule.preferred_amount_max, class: 'form-control' %>
    </div>
  </div>
</div>

Swap select2 for bootstrap custom-select.
Swap select2 for bootstrap custom-select.
