What is needed commercially and legally?
A payment gateway application takes a variable time and asks for company documents, so it has to start alongside the build. Working through a payment provider is faster but carries a different per-transaction fee.
On the legal side, distance selling terms, return and delivery conditions, a privacy notice and a cookie policy have to exist before launch. These are not templates to be filled in; they are written around what you sell and how you deliver it.
Why is product data the critical item?
Because it sets both how the store looks and whether it is found. A shop opened with missing product names, copy-pasted descriptions and low-resolution photographs neither sells nor ranks.
Variant structure is settled here too: size, colour, measure and bundle combinations shape the data model directly. Changing it later means entering the catalogue again.
Which integrations belong in the first conversation?
Four: payment, shipping, invoicing and accounting. Every gateway and every carrier is a separate integration, and how many get connected grows the scope directly.
Then marketplaces. If stock and prices go to external channels, each is counted separately, and whether the link runs one way or both is a decision of its own.
What has to happen after launch?
Day one is a beginning, not an end. Submitting the sitemap, setting up measurement, watching the abandoned-basket flow and checking the first orders by hand all belong to this period.
Most problems in the first month are not software faults but missing decisions: how returns are handled, what happens when a carrier damages goods, whether orders are accepted when stock runs out. Unsettled before launch, they get asked in the first week.