I read a few articles including: 7 Tips for Load Testing an E-Commerce Site and 4 key factors to consider in performance testing of E-commerce applications.
The first includes a list of things to check, as well as some caveats. This includes payment processing, email functionality, inventory, and back end integration.
The caveats I think can be summed up as follows: When you are load testing, don’t create more work for yourself, or place undue burden on the systems you are NOT supposed to be testing as a result. For example, if you are testing e-mail functionality, you should probably have all the emails sent to the same address, or you can imagine the cleanup you might have to do afterwards!
Also, don’t tick off your payment processor! Load testing on their system might get you shut out. In theory, your payment processor has already load tested their systems - but if you must, make sure you make the appropriate arrangements with them.
Anyway, see the full article for the details.
The second article is a higher level overview including more advice about making sure you check across geographies, include mobile and cloud based testing, and test across all browsers and devices. I don’t think this information is really specific to load testing, but it is good to keep in mind.