LONDON (Reuters) -Heavy online marketing spending by Temu and Shein is making it more costly for other retailers and brands to reach shoppers on Black Friday, marketing and industry experts say, with both platforms bidding heavily on search keywords used by competitors. Typing a few words into a search engine is a key starting point for shoppers looking online for gifts or buying for themselves in Black Friday sales, the unofficial start of the holiday shopping season on the day after U.S. Thanksgiving. Retailers compete for their advertised products to appear high up in online search results, by bidding on keywords.