Weigh a strip of 100 of the CRB25 type and similarly for the CRB50 type, and you'll have a good enough mass per resistor to work out the postage weight. given the masses, the type and estimated quantity remaining of each value, the mass of your plastic bags and padded envelopes, it would be fairly simple to set up a spreadsheet to tell you how many to cut and pack of each value to come to just under any desired postage weight, without exhausting the stock of any value prematurely.
Once you have a packing list, it shouldn't take more than an hour to cut and pack 20 bags, and figure another hour for envelope stuffing, labelling and stamping + whatever time it takes to make a post office run.