The work between the systems
Manual order entry is not one task. It is a chain: a call, an email, a spreadsheet, a price check, a stock check, a credit check, a data entry step, and a confirmation. Each handoff adds time and a chance for the order to be wrong.
A simple cost model
Start with the number of orders that arrive outside the website each week. Multiply by the minutes spent per order. Add the cost of corrections, expedited shipping, and the sales time spent on status questions. The exact numbers will be yours; the pattern is usually familiar.
The bigger cost is often invisible: the customer who stops trying to self-serve because the website cannot see their price or their contract.
What changes when the order arrives clean
When the online store knows the account, the contract, the stock, and the terms, the customer can place a clean order without a phone call. The ERP receives the order in the same shape as a rep-entered order, with fewer problems.
That is the operational case for an ERP-connected B2B eCommerce site: not a prettier website, but fewer manual steps between the customer and the main system.