Custom Retail Software: Multichannel Stock Without Mismatches

You sell in a physical store, on your own website and on a marketplace, and the stock never adds up. The website sells a size that sold out in store yesterday, Friday's count does not match the ERP, and someone spends 6 to 8 hours a week reconciling figures by hand. Custom software that keeps POS, ecommerce and ERP in sync costs between 3,000 and 10,000 EUR and removes the mismatch at the root.

Why does stock drift when you sell in store, online and on marketplaces?

Stock drift is not bad luck, it is architecture. Each channel keeps its own inventory: the POS deducts when you charge at the till, the website deducts when an order is confirmed, the marketplace reserves instantly and the ERP finds out overnight through a batch import. With three or four systems syncing every 24 hours, one day with 20 or 30 cross-channel sales is enough for the website to sell the last unit that left the store at midday. The customer pays, there is no product, and you have to cancel the order.

The consequences show up in money and in hours. Amazon penalises sellers whose cancellation rate climbs above 2.5%, and every out-of-stock cancellation pushes you towards that threshold. A retailer with two stores and an online shop easily spends 6 to 10 hours a week reconciling inventory by hand, around 400 hours a year. Then comes defensive overstock: since nobody trusts the numbers, you buy extra, and that cushion is cash sitting frozen on a shelf.

What should multichannel stock software actually synchronise?

The central piece is a single inventory ledger that every channel reads from. Each movement updates that ledger within seconds: a till sale, a web order, a marketplace reservation, a return, a transfer between stores, goods received from a supplier. A reasonable latency target is under 60 seconds between a sale happening on one channel and the rest showing corrected stock. Overnight batch syncs, still the norm in many small retailers, always arrive too late to prevent the next mismatch.

You do not need to replace what already works. The custom layer talks over APIs to your POS, your ecommerce platform (Shopify, WooCommerce, PrestaShop or a custom store), marketplaces such as Amazon or Miravia, and ERPs common in Spain like Sage, Holded or Odoo. It is the same approach we apply in ERP and CRM integration work, where a typical integration lands between 1,500 and 4,000 EUR.

How does the POS, ecommerce and ERP integration work in practice?

Most current POS systems and ecommerce platforms expose REST APIs or webhooks, so the pattern is direct: every sale, return or goods receipt raises an event, a central queue puts events in order and applies them to the single ledger, and retries guarantee that no movement is lost when a system goes down. That queue matters more than it looks: without guaranteed ordering, two simultaneous sales of the same SKU will drift the stock again. No CSVs sent by email, no syncs at three in the morning.

What if the POS is old and has no API? There are three ways out: read its local database, automate exports every few minutes, or replace just the till module, never the whole platform. In a project for a fashion chain with three stores in Barcelona we kept the existing POS, connected it to the online store and the ERP, and SKUs with wrong stock dropped from 14% to under 2% in five weeks of work.

Which replenishment rules prevent stockouts and overstock?

Once stock is unified, the next step is letting the system decide when and how much to replenish. The basic rules: minimum and maximum per SKU and per store, days of cover calculated from real rotation, and alerts when a SKU falls below its threshold. From there, the software generates supplier order proposals and transfers between stores: if store A holds 9 units that are not moving and the website is selling them, it proposes moving 6 before buying more.

For marketplaces, publish available stock minus a safety buffer of 1 or 2 units per SKU; that simple rule takes out-of-stock cancellations down to practically zero. The financial effect is measurable too: small retailers that move from gut-feel purchasing to cover-based replenishment typically cut frozen stock by 15% to 25% within the first year, cash that becomes available again to buy what actually sells.

How much does multichannel stock software cost and how long does it take?

The usual range for a multichannel stock project is 3,000 to 10,000 EUR, depending on the number of channels, stores and systems involved. A simple integration between two systems, say POS and ecommerce, costs between 1,500 and 4,000 EUR and ships in 3 to 6 weeks. A full project with POS, ecommerce, two marketplaces, ERP and replenishment rules sits near the top of the range and takes 2 to 4 months, delivered in phases so synchronisation works from the first weeks.

The terms matter as much as the figure. We work with a fixed price agreed before starting, so the budget does not grow through unplanned hours. The same senior engineer who analyses your case and quotes it is the one who delivers, with no subcontracting. And the code is yours from day one: if tomorrow you want another team to maintain it, you can. For ongoing evolution there is a technical retainer from 500 EUR per month, optional, never a compulsory fee.

Custom ecommerce or Shopify when you already run a physical store?

Shopify or WooCommerce are a reasonable way to start selling online, and there is no point dismissing them on principle. The problem appears when the website has to share inventory with the POS and the ERP: generic sync apps cover the standard case and fall short as soon as per-customer pricing, multi-store stock or your own replenishment rules enter the picture. We compare both routes with concrete costs in custom ecommerce or Shopify.

The practical decision is less dramatic than it sounds. If your online operation is standard, keep the current platform and build only the synchronisation layer with POS and ERP, between 1,500 and 4,000 EUR. If you need your own logic, such as B2B price lists, click and collect with per-store stock or in-store reservations from the web, a custom ecommerce build from 5,000 EUR saves you fighting platform limits every time the business changes.