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This is controlled by the ALG PS parameter. By default, position 20 is blank, so pieces of both weights go into the same tray and get the same 'blended' rate.
If ALG PS position 20 has an S, then the different weight pieces go into separate trays (by ounce weight), and then will pay the rate based on their own weight.
If ALG PS position 20 has an S, then the different weight pieces go into separate trays (by ounce weight), and then will pay the rate based on their own weight.
UPDATED:
May 14, 2019