Hi Nan,
Please review the following documentation on adding sales pricing. Here is a message regarding this from Rakuten:
Concerning the use of sale pricing, it is true that you don’t have the option via the portal but you do via the feed option. Please reference the BBY 5.0 inventory feed spec (I have attached). There are 2 pricing fields: Price and Original Price. The use of an ‘Original Price’ that reflects a higher price than ‘Price’, invokes the ‘sale’ logic on the BestBuy site. Please let us know if you have any further questions. Thanks!