The concept of the dopamine reward system is well known and often used in training areas in order to keep consistency high, this is a process that follows anticipation, action and reward.  Dopamine is an amazing transmitter your body creates using the amino acid tyrosine which converts to an acid called L-dopa, which enzymes turn into dopamine.  From this state, the nerve cells can send messages throughout the body which contribute to many bodily functions such as blood vessel function, heart rate, kidney function, and sleep among others.  These functions it also contribute to behavioural conditions such as mood, motivation, learning and pain processing.  Due to its core functions, it is a key contributor to the anticipation, action, and reward cycle.  What research has shown is an increased release of dopamine associated with the anticipation of a reward.  What is commonly misinterpreted is that the release comes at the time of the reward, however, results in Psychology Today revealed that the initial signal once an action is decided releases 50% more dopamine during the anticipation to action before the reward.  This was attributed to the uncertainty or predictability of the odds.  

In the case to explain consumer shopping habits, this can be applied to online shopping as the dopamine release is at the checkout, throughout the anticipation of your items arriving, then the reward once they do, however the reward stage of receiving the goods is the uncertainty applied.  If you often expect to receive perfect fit, sizing, colour and your expectations are met then you are satisfied, however, should you receive the product and be let down by expectation your mood may shift.