code: public PaymentSummaryBuilder setPaymentSummary(String currency,
  			PaymentType paymentType, String merchantName) {
  		return new PaymentSummaryBuilder(currency, paymentType, merchantName,
  				this);
  	}
docs: Sets the {@link PaymentSummary} for the current {@link BuyButton}.
  
  @param currency
  the {@link PaymentSummary#currency}.
  @param paymentType
  the {@link PaymentSummary#paymentType}.
  @param merchantName
  the {@link PaymentSummary#merchantName}.
  @return this builder.