If you need Alibaba to do drop shipping, you can provide authorization letter and purchase certificate in the following ways. First, you can contact the supplier or manufacturer and ask them to provide an authorization letter, clearly authorizing you to act as an agent for the sale of their products. Second, you can ask the supplier or manufacturer to provide purchase certificate, such as a purchase contract, invoice, or payment voucher, to prove that you have purchased these products. You can scan or take a photo of these documents and send them to Alibaba via email or an online file sharing platform for their review and confirmation. Make sure the documents are clear and contain necessary information, such as supplier name, product description, quantity and price, etc.