First of all, who is your dropshipper? Is it someone you found online or a local supplier who has agreed to dropshipfor you..If it is someone online, then most likely you will sign a contract or 'user-agreement' and when you select 'agree' then that will become your contract between you and the online shipper...If it a physical store front that you are doing business with then by all means have a contract drawn up and signed by both parties...
Buying any product site unseen is a risk anyway you look at it. This is why I don't recommend dropshipping. I think it is a fast way to kill ones business. The main reasons being is that you as the seller have no control over the actual quality of the product, or the shipping time, and things of that nature...That being said, some suppliers who do have authentic goods, will back those goods with an 'authenticity gauranteed' clause in the contract. Others might be willing to produce a 'sanitized' invoiced to prove authenticity...
Too, keep in mind that there are no designer goods that you can buy from a wholesaler that are authentic. Designer companies do no wholesale their goods...Some do allow 'sub-licences' to be granted from certain distributors, and of course you can always get deisgner goods through closeout stores, liquidation sales and salvage/surplus companies....but other than those places, anything else you get will probably be fake...