At one end of the scale, you could put up a website in a day and sell invitations the next day, and all you'd need is somebody who would find your website and want to pay you money for your invitations. You can do that tomorrow for a few dollars and a few hours ... and, of course you'll get very few, if any, buyers.
At the other end of the scale, you'd need not just a website but a complete marketing strategy to generate traffic on that website, deliver a message, convert browsers to buyers, take credit cards and produce and ship your invitations in volume sufficient to meet demand.
At the high end, a competitive website is a big deal, and search engine implementation, buying adwords and such is a big deal. Generating the processes to produce invitations well and quickly and in sufficient volume is also a big deal. Being able to take credit cards is a legal hurdle and a technological hurdle. Building the website is a technological hurdle and a marketing-sales hurdle, taking expertise, time and money.
This is why people suggest developing a plan. In the end, you look to answer the main questions yourself, based on your specific situation. It doesn't have to be a full formal business plan document, like you would turn over to an investor or a bank; but you do have to investigate the loose ends and make some real decisions.