Ballot harvesting is the practice that allows third parties to collect ballots for drop-off at polling places and election offices. What is to keep the third party from changing the ballots or destroying them? California was the first state to allow ballot harvesting.
An example, on election night in 2018 the California Central Valley Congressional race was called for the Republican. The incumbent had a 5,000 vote lead. The ballot harvesters brought in additional votes and eventually the Republican lost by 862 votes. Tracking vote harvesting in California, it should be noted that Republicans have lost every Congressional seat in historically Republican Orange County.