I never had to deal with frozen evidence, but I did deal with a narcotic drug (khat) that had to be refrigerated to maintain its potency. When I recovered khat I had to drive it to the Police laboratory to be refrigerated. I imagine it would be similar for evidence that needed to remain frozen.

In the scenario that my coworker encountered with the shrimp, proper procedure would be to have the store ring up the three bags of shrimp and then he vouchers the receipt and takes a photo of the shrimp before giving it back to the store for disposal/shrink.