Keep in mind that those with a PO box almost always have a physical address that UPS (and other couriers) can deliver to.
UPS does charge based on total weight of all packages in a shipment vs CP which charges based on the weight of each package - this can save you considerable money if you send multiple items.
As far as basing the discounts on weight vs. dollars spent - the two are interchangeable essentially, right? I can't comment on what their internal workings are - but they'll ask you about package weights, # of packages, service levels, destinations, etc. in order to provide a quote.
UPS discounts are also setup according to "target" - so you might be 50% off at 100% of target, 40% at 80% of target, 30% at 70%, etc. This is based on a 12 week period I think to it evens out spikes.
UPS insurance is similar to others in my experience - they want to only pay on the shipper's cost and will only offer insurance on new items. This of course is the policy for the "masses". Everything is open to negotiation...
$40K or more per year in freight with UPS will get you into their pca program - dedicated account rep (no more dealing with the standard 800#) plus email support for everything - which I find incredibly handy.