While each option (Separate Site or WordPress Plugin) has its strengths and weaknesses, using a separate site tends to be a better option if you focus solely on commerce. A separate site is preferable because it provides an easy templated system that is dedicated to the sale of digital or physical goods. All of the costs associated with your commerce page are visible upfront, which makes budgeting easy. A standalone site will also handle security solutions that are tailored to payment transactions. An integrated site can also require the modification of some code, so some coding knowledge may be required. Lastly, many standalone sites support both online and in-person transactions.