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If you are purchasing these items to gain more customers you would list them as advertising or promotions.  This applies whether you keep the items for the next event or give them away during an event. 

If you are buying items that are meant directly for use with the client then that would be supplies.  

Overall, it will not have an affect on your return which category you use if you label a few dollars to the one side versus the other because it can be used for both.  Both of these expenses are deducted $1 for $1 and lump summed into the total expense deduction.