As well as an understanding of PCI/DSS, there are Distance Selling Rules which are applicable to trading over the internet. These work in conjunction with the the Sale of Goods Act and the Data Protection Act.
Online shops selling physical product have different carriage needs. Charge too much or make your carriage too complicated, and you'll confuse or lose your customer. Charge too little and you'll make a loss.
In terms of your website: If you're using an off-the-shelf shop package, be clear on what carriage features could be in place and how they will work.
If you're developing a bespoke shop, make sure you're as detailed with your carriage needs as possible.
Launching your new website and being number one in Google the very next day for every key phrase you can think of sadly isn't going to happen. With this in mind, you're almost certainly going to have to advertise.
More often than not, the real cost and effort of developing an online shop is after the shop is built, to before.
Advertising such as Google AdWords, Price Runner, Shopzilla, Google Products and Kelkoo are common. You may not be able to ascertain what will work and what won't, but with some thorough investigation, you will be able to allocate a realistic marketing budget and ensure you're focusing on the right products.