One way would be to have a table called e.g. “deliveries” with “units received” field. It would be linked to products table which would use a Roll-up type field: Total Received. This field would use the SUM(Values) function.
Do you need a running total across multiple rows? Or is only a final total okay?
If you need a running total, you can adapt the running total script. Add the units received to the total received from the previous record to get the new total received. Continue down the rows.
If you only need the final total, go with Matt’s suggestion of linked records and a rollup field.