Launch is officially Friday, April 1, so Best Buy could easily ship your phone out tonight or tomorrow and it could easily reach your door step on Friday.
The carriers are selling many more phones than Best Buy so it's easy to reason that they would get priority on shipments from LG and with more orders to ship, they would get started as soon as they could.
Many times on street dated launches for phones, tablets, video games, etc, retailers get their shipments drop shipped straight from the manufacturer to the stores, bypassing the retailer distribution centers all together. In this case, the warehouse that ships out internet orders would be very similar to a store. They would be waiting on their shipment as well.
I ordered my G4 from a Best Buy store. When I walked in at 10am to pick it up on launch day, they hadn't received the FedEx shipment yet. I had them call the store 5 miles away and they had 'plenty' so I went there and got my phone.
Even with an in store preorder of the G4 I was told it wasn't a guarantee they would have it on launch day.