Snipcart docs and support feel all over the place on this. Release notes for v3.1.0 explicitly state that <summary-fees> is overridable, as does the source code (as of 3.4.0)
return n('overridable', {
        attrs: {
          name: 'summary-fees'
        }
      }, [ etc...
As far as I was able to cobble together based on source code, this seems to be the default markup (using a modified version of  component listed in the v3.0.13 default theme guide as a baseline):
<summary-fees
            class="snipcart-summary-fees--reverse"
            :summaryData="summaryFeesProvider"
          >
              <div class="snipcart-summary-fees__notice snipcart__font--regular" v-if="!summaryData.hasShippingAddress">
                <slot>
                  {{ $localize('checkout.shipping_taxes_calculated_when_address_provided') }}
                </slot>
              </div>
              <div class="snipcart-summary-fees">
                <div class="snipcart-summary-fees__item snipcart-summary-fees__discounts snipcart__font--slim" v-if="summaryData.hasDiscounts">
                  <span class="snipcart-summary-fees__title">
                    {{ $localize('cart_summary.discount') }}
                    <div class="snipcart-summary-fees__discounts-icon" v-if="summaryData.discounts">
                      <icon name="question-mark" @mouseover.native="displayDiscounts = true" @mouseleave.native="displayDiscounts = false"></icon>
                      <tool-tip v-show="displayDiscounts" class="snipcart-tool-tip--bordered snipcart-summary-fees__tool-tip">
                        <div class="snipcart-summary-fees__discount-container" v-for="discount in summaryData.discounts" :key="discount.id">
                          <div class="snipcart-summary-fees__discount-name snipcart__font--black">{{ discount.name }}</div>
                          <div class="snipcart-summary-fees__discount-amount-saved snipcart__font--black">-{{ discount.amountSaved | money(summaryData.currency)}}</div>
                        </div>
                      </tool-tip>
                    </div>
                  </span>
                  <span class="snipcart-summary-fees__amount">{{ summaryData.discountsTotal | money(summaryData.currency) }}</span>
                </div>
                <div class="snipcart-summary-fees__item snipcart__font--slim" v-if="summaryData.subtotal">
                  <span class="snipcart-summary-fees__title">{{ $localize('cart_summary.subtotal') }}</span>
                  <span class="snipcart-summary-fees__amount">{{ summaryData.subtotal | money(summaryData.currency) }}</span>
                </div>
                <div class="snipcart-summary-fees__item snipcart__font--slim" v-if="summaryData.hasShipping">
                  <span class="snipcart-summary-fees__title">{{ $localize('cart_summary.shipping') }}</span>
                  <span class="snipcart-summary-fees__amount">{{ summaryData.shippingInformation.cost | money(summaryData.currency) }}</span>
                </div>
                <div class="snipcart-summary-fees__item snipcart__font--slim" v-for="tax in summaryData.taxes" :key="tax.name">
                  <span class="snipcart-summary-fees__title">
                    {{ summaryData.tax.name }}</span>
                  <span class="snipcart-summary-fees__amount">{{ summaryData.tax.amount | money(summaryData.currency) }}</span>
                </div>
                <div class="snipcart-summary-fees__item snipcart-summary-fees__total snipcart__font--bold snipcart__font--secondary">
                  <span class="snipcart-summary-fees__title snipcart-summary-fees__title--highlight snipcart__font--large">{{ $localize('cart_summary.total')}}</span>
                  <span class="snipcart-summary-fees__amount snipcart-summary-fees__amount--highlight snipcart__font--large">{{ summaryData.total | money(summaryData.currency) }}</span>
                </div>
              </div>
          </summary-fees>
It would be great if some attention could be given to the documentation as to what is and is not truly overridable.