Go back in with your receipt they will refund or give you another one.
They don't have a direct line, take the receipt back and ask for a refund, or cough up the roast.
As already mentioned try going back with receipt & explain you didn't have it & hopefully the will work it out for you. I have found the staff to be very helpful. Things move rather fast so I'm sure you will be on the lookout for things being misplaced.
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