This only tends to be an issue for larger businesses with many product lines, where separated orders and payments cause administrative headaches.
If you have an existing computerised stock control and accounting/ordering system, you may well need to integrate them into the web sales process, so customers can see whether the item they want to order is in stock, and your ordering and accounting arrangements are maintained in real time as well.
This sort of work is outside the standard Web design service we offer, however we will support, advise, and can help clients to identify a suitable specialist e-commerce ISP to undertake this work for them. We can and work with them to integrate your existing website if you have one, or to design one that is compatible with the sales option you select if you opt for an all singing, all dancing e-commerce site now. Typically the e-commerce bolt-on might cost a thousand pounds to establish, and is likely to have ongoing costs as well.
For example, Cable & Wireless were offering secure services for small to medium sized businesses on a subscription basis with a £1,000 set-up, and £1,000 per year, plus 5% of each transaction. These charges include fees for payment and order processing, but not web hosting or your merchant agreement which are extra.