Storefront or back office
An online store answers a question about demand: how do people find you and order. A billing system answers a question about operations: how does the sale get recorded, what happened to the stock, who owes you money, and did the material reach the site. A shop can need both, and neither replaces the other. Being clear about which problem you are actually trying to solve saves buying the wrong one.
Does the material trade sell online at all?
Some of it does. Sanitaryware, tiles, paint and hardware have real online demand, particularly for a homeowner comparing before visiting. Cement, steel and aggregate mostly do not — the buyer is a contractor who wants a rate on the phone and a load by afternoon, and price moves too fast for a catalogue. Be honest about which half of your business an online store would actually serve.
What still has to happen after the order
However the order arrives, a GST invoice has to be raised at the right rate, stock has to come down, the credit has to go onto the party's account if it is not a cash sale, the load has to be recorded with a vehicle number, and the money has to be collected. That is the work an online store hands over rather than doing, and it is the part that decides whether the shop makes money.
Running both
There is nothing wrong with taking enquiries or orders online and billing them in a shop system — plenty of hardware and sanitaryware shops do exactly that. Treat the storefront as a source of enquiries and the billing system as the record of what actually happened, and neither is doing the other's job badly.