It is so much cheaper and more efficient than traditional storefronts
Arguable. A local store gets a relatively large shipment of merchandise to sell, so the cost (in terms of money, environmental damage, traffic, etc.) of shipping is spread over the entire lot. A customer may make several purchases (including from neighboring shops) on a single trip, which spreads the cost of driving over the number of items. Some customers might even walk or bike to the local store, or stop by on the way home, further minimizing that segment of the cost.
In the on-line version, if you buy everything from Amazon, in some cases they can hold shipment until all the items are complete, but in many cases even stuff you purchased at the same time are shipped separately. If you don't buy everything from Amazon, or if the order actually goes to some associate/affiliate (whatever they call it, those third party merchants) of amazon.com, then certainly they will ship separately. If you and your neighbor both buy something on-line, but one ships UPS and the other FedEx, then there'd be two trucks coming to your block instead of one. Not terribly efficient at all.
if people are forced to pay sales tax on purchases [...], then that is going to lower the economic incentive to purchase online.
Do you realize that this statement could not be true unless on-line shopping is actually less efficient and more expensive? If on-line shopping is really "so much cheaper and more efficient" even after taxes were properly assessed, why would anybody stop?