Trying to charge me

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Hey all, I had used Uber to order some groceries almost a month ago, using a uber promotion that was 'enjoy 50%off up to $40'. So before the promotion my cart was $44.47, after promotion it went down to $23.50, plus the tip for the driver the total order amount was $26.16. Unfortunately one of the items I ordered was not available so that was "credited" back, lowering my total to $18.92. But since that order uber has tried to charge me 7.27 repeatedly rather than refund me bc I their system "no promotion was applied". Ive tried support but its going nowhere, what else can I do?