Your e-commerce site must process transactions correctly 100 percent of the time. No exceptions. Customers will not tolerate failed
transactions. Besides a tarnished reputation and lost customers, you
may also incur legal liabilities associated with failed transactions.
You can consider transactional testing as system testing of the business layer. In other words, you test the business layer from start to finish, trying to uncover errors. Once again, you should have a written
document specifying exactly what constitutes a transaction. Does it
include a user searching a site and filling a shopping cart, or does it
only consist of processing the purchase?