An online store does three jobs at once, and they are separable.
1. The shop window. Showing what you sell, so somebody can decide.
2. The till. Taking the money.
3. The warehouse. Knowing what you have left, what is reserved, what has shipped.
You are quoted for all three because "online store" means all three. But a good half of the businesses that ask for one only need the first two.
When you only need the till
If you sell a handful of things, or the same thing repeatedly, or you agree the order in a conversation and only need paying — you need a payment route, not a store. A page that describes the work and a link that takes the money is a smaller build, a smaller monthly cost, and less to go wrong.
The tell is stock. If you never say "sorry, that's gone", you do not have a warehouse problem, and you should not be paying for warehouse software.
When you genuinely need the warehouse
You need it the moment two customers can buy the last one. Everything expensive about a store is downstream of that single fact: reserving stock while somebody is checking out, releasing it when they abandon, keeping the number right when a refund happens, and keeping it right across a shop and a counter at the same time.
If you sell physical things in limited numbers, pay for this. Trying to run it on a spreadsheet beside a payment link is how a shop oversells its last item on a Friday and spends Saturday apologising.
The middle case people miss
Some businesses need the shop window and the warehouse but not the till — a trade supplier whose customers order on account and are invoiced monthly, for instance. That is a real shape and it is often cheaper than a full store, because payment is the part with the most rules attached.
Two questions worth asking before you buy
"What happens when I change a price?" In a real store, an order already placed keeps the price it was placed at. If a supplier cannot tell you where that is written down, the price on your invoices will drift from the price on your page.
"What happens on a refund?" Not "can we refund" — every system can. Ask what happens to the stock number, the invoice, and your bookkeeping. That is where the difference between a real system and a form shows up.
And one thing that is not optional
Whatever you buy, you are selling to consumers under UK distance-selling rules, which give the buyer cancellation rights and require certain information before they pay. That is not a feature to add later; it changes what the checkout has to say. Ask how it is handled before you sign anything.