<%= f.field_container :volume_price_model, class: ['form-group'] do %>
  <%= f.label :volume_price_model_id, I18n.t('spree.volume_price_model') %>
  <%= f.collection_select :volume_price_model_ids,
    Spree::VolumePriceModel.all, :id, :name,
    {
      include_blank: I18n.t('spree.match_choices.none')
    },
    {
      class: 'custom-select',
      disabled: cannot?(:edit, Spree::VolumePriceModel)
    } %>
  <%= f.error_message_on :volume_price_model %>
<% end %>

<div class="row">
  <div class="col-4">
    <%= f.field_container :volume_price_model, class: ['form-group'] do %>
      <%= f.label :volume_price_model_id, I18n.t('spree.volume_price_model') %>
      <%= f.collection_select :volume_price_model_ids,
        Spree::VolumePriceModel.all, :id, :name,
        {
          include_blank: I18n.t('spree.match_choices.none')
        },
        {
          class: 'custom-select fullwidth',
          disabled: cannot?(:edit, Spree::VolumePriceModel)
        } %>
      <%= f.error_message_on :volume_price_model %>
    <% end %>
  </div>
</div>

Put volume_price_model select into grid
Put volume_price_model select into grid
