I recently came across a design in the MOC pop-up store that interested me, and thought I'd share my experience thus far...
The design holds ~700 unique elements, and ~3200 pieces over-all. The build is comparable to a $300 production set, so I decided to pursue it. Unfortunately, so far, the auto-cart mechanism is falling waaaaay short. On the model page it estimates a price of ~$480. When I click on "Purchase", it claims first off that Lego's Pick-a-Brick is not available for this model. It then gives me three options: international (lowest piece price) $580; fewest orders $745; US stores only $690. None of these estimates include shipping/handling.
Bricklink doesn't give the option to exporting the parts list, but one can populate the carts and export those. I merged them into single wanted list, and priced it out in brickstore where the total cost based on average part sold price is ~$345.
In brickstore, one can sort by either per-piece cost, and total lot cost. So far, I've found 28 of the highest-cost parts on PaB (208 total pieces), where the savings is ~$22 ($43 on PaB w/ free shipping, $65 based on bricklink sold prices w/o shipping costs included).
So, I'm a 4% of the way through the parts list, and have already saved ~40% on some of the highest cost items by going through LEGO PaB. Presuming I can get the rest on bricklink in 4-5 stores, I can probably source the parts for under $400.
I must say, I'm a bit disappointed. The dream of pushing a few clicks and then having a MOC show up in 3-4 boxes is appealing, but the higher cost to me doesn't justify relying on the automation. And if the buyer has a significant spare parts inventory, then it makes sense to go through to carts/parts list anyway to further reduce the cost. I find it's easier to do this through brickstore and wanted lists rather than in-cart.
I'm rating the MOC pop-up store a 'miss'. The biggest miss is that the automated cart chose to skip Lego's PaB, even though many of the most expensive parts are available there.
Haven't yet decided if I'll go ahead an order it. If I do, I'll post an update with the final cost.