I don't know or understand the advanced load thing but I heard tonight that our UPS Store had over 400 pcs going out today. Some of these were P/U'd on sat but the rest went out today. They had to have 3 p/u's today.
When I managed Foothill Center in SEC, we had about 20 mail box stores. Starting the Monday after Thanksgiving, I would send out two 24 foot vans with helpers starting at 10 am to prelim all the mailbox stops. We would fill both those vans two times each with just the stores!
It was great for the twilight operation because they could pre-charge the operation with this volume.
Advanced loads usually come in on the rails prior to the scheduled delivery date. Normally the loads sit unless volume is expected to be light and then the loads are advanced or moved up and unloaded prior to the schedule.
At Xmas this becomes a standard practice to keep the network from being overloaded. Some districts will advance as much volume as they can handle to make the coming days go a little easier (or not blow up the capacity).