One of the main decisions is on the governance of the project: how do you inform management, how do they choose and steer the project? How do you manage the development? What is the route in case of problems and issues? How are requirements set?
There are two main choices to be made in a medium to large scale undertaking: Project/programme governance - which deals with the stakeholders, sponsors of the project and external parties. IT delivery method - defines the way that you work with your team.
The easiest way to define the governance is to use the existing project governance within the organisation. This means that there is consensus on wording, there is experience and the specific rituals are already embedded. PRINCE2 is one of the most known project management methods.
For smaller projects, the choice of the IT delivery method is the major choice, as governance should not be the issue and can be dealt with within the team.
Agile methodologies are mainly the way to go, as developments in ecommerce require short cycles. An important consideration is how the project deals with external factors that the project needs e.g. Data, pricing, logistics.
A few examples (but not all)
Are the back-end systems already in place and/or adjustment needed? Most of these systems have a longer development and release cycle. What is the degree of connection? Are prices, products and stock added manually or automated. Are they updated in real-time? The more connection, the more dependencies there are and the more chance that changes are needed during the project.
SCRUM or Kanban can be used when the external dependencies are low or when they have a similar development cycle. When there are more dependencies and/or a longer development cycle, DSDM might suit more, as it takes more time in the first phases to explore the dependencies further. Traditional methods might suit when the external systems have a strong influence, a lot needs to be developed end to end to get the platform to perform.