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Antieverything touched on the cruel reality of this situation: -Unless you can
prove that the old pedal was sound (not stripped/damaged/worn from use) - "it is their word against yours", and the credit card company will likely side with the shop. - After all, you agreed to purchase a replacement.
I might take the original pedal back to the old shop near the old 'hood for an assessment: If the pedal is still sound, and they're willing to put it in writing, then you have a case, given that it is incumbent on a LBS to insure that nothing is defectively-fastened in the course of a tune-up.
In any case, I would be inclined to make that small investment in tools and begin doing your own maintenance, unless of course you have no interest in DIY, and it is practicable to continue to patronize the now more distant shop which you know and trust.
PS - If you do end up writing to the credit card company to make your case - keep it short!
